When I reviewed Oyster six weeks ago, I was impressed by its design and its offerings — over 100,000 in-copyright ebooks for $9.95 a month. But I thought the app’s lack of availability on iPad was a big drawback because I think it’s hard to do serious reading on an iPhone. The launch on iPad remedies this problem, of course, and subscribers’ books will sync between both devices.

Oyster’s design on iPad is great, just as the iPhone app design is. One feature I liked is that you can tap to turn pages on the iPad app — it’s a gesture that anyone who’s used a touchscreen e-reader should be familiar with, and it’s easier than swiping.

Oyster CEO Eric Stromberg and cofounder and chief product developer Willem Van Lancker wouldn’t tell me how many people have signed up for Oyster in its first six weeks. Van Lancker said the reaction has “exceeded all our expectations” and offered a semi-statistic: Ten days into launch, users had read 1 million pages; the second million pages were read in six days; and the third million were read in three days. Stromberg said that “in the initial push we saw a significant level of enthusiasm that translated to sign-ups” and that growth since then has been “very consistent.”

Oyster has also added some more content since launch. A few new publishers have joined — including “a couple small prestigious independents” — but the company wouldn’t name names. Stromberg said that many of the publishers who initially signed up with Oyster have added “lots of new titles” to the service since it launched, for example, “HarperCollins sent us a bunch more titles.”

Stromberg mentioned HarperCollins because, about a month ago, document-sharing site Scribd publicly launched its rival ebook subscription service, for $8.99 a month and with almost all of HarperCollins’ backlist (books over a year old). HarperCollins explained to me at the time that it chose to make more titles available to Scribd than to Oyster because Oyster is a startup, while Scribd, which was launched in 2007, already has about 80 million users and “the hardest thing is building audiences at scale.” Another bonus for HarperCollins was the fact that Scribd is available across platforms, including Android.

Scribd‘s ebook subscription service has been up and running quietly since January, and this week the company shared a few stats that provide some idea of how subscribers are using ebook services. Scribd’s “power user” reads about 10 books a month (though, according to information Scribd presented at the Frankfurt Book Fair, “power users” make up only about 2 percent of subscribers). For every book read completely, the company said, a user browsed an average of 4.5 other books. And check out the most popular devices for reading:

That reiterates the importance of Oyster launching on iPad.

Eric Goldman and Rebecca Tushnet are both trademark experts and popular bloggers. This week, they offered a new package of course work called “Advertising and Marketing Law: Cases and Material” for sale on Scribd for just $10. In a blog post, Goldman explains the economics behind the $10 price tag:

Compared to 99 cent or free eBooks, a $10 downloadable book may sound expensive. But, compared to the typical law school dead-trees casebook, $10 is a ridiculous bargain. Many print casebooks of comparable size cost $150 or more. … While we could easily justify a higher price than $10, we’re not exactly philanthropists. Here’s how I see the math: a $150 casebook may have a $110 price wholesale (or less). At 10% royalties to the authors, Rebecca and I would share $11. At the $10 download price, Scribd takes $2.25 a download, leaving us author royalties of $7.75. So discounting the retail price 93% perhaps reduces our royalties by less than 30%. Let’s hear it for disintermediation! Plus, just like any demand curve, the lower price point should lead to higher sales, which may, in fact, make our approach profit-maximizing.

The book will be available to read and mark-up on any electronic platform and can be printed as well. The authors are not applying any digital rights management to the book, but are relying instead on the low price and a moral appeal to limit unauthorized sharing.

The book is available on Scribd, a popular document-sharing platform, that has already attracted the interest of the publishing community. In 2009, Simon & Schuster signed a deal to use Scribd as a potential Kindle competitor, offering excerpts and complete works from Stephen King and others.

Interestingly, in his blog post, Goldman blasts Scribd as a “horribly limited platform” and complains that it requires users to provide their Facebook profile to complete the purchase. He adds that he and Tushnet intend to switch to an alternative eBook platform as soon as they can find a better one.

Goldman and Tushnet are part of a larger phenomenon of disruption in the textbook publishing business that is leveraging digital distribution possibilities and eliminating intermediaries. Other examples include sites like BenchPrep and Boundless Learning that offer personalized or open source content via mobile and the web.

Float is debuting seven months after Scribd raised $13 million in a third round funding intended to support its mobile plans and new products.

While mostly known for document sharing (paidContent and others often use the platform to post legal documents and company presentations), San Francisco-based Scribd has been aiming itself towards a wider consumer market with an eye toward finding a middle ground between publishers’ native apps and PDF-style versions of websites for reading on tablets like the iPad.

Float represents the clearest form of Scribd’s attempt to expand its position as a document sharing company to one that has aimed much of its services at major book and magazine publishers as e-readers and tablets have become mainstream products.

Mobile, naturally, has been at the center of that strategy since early 2010, which it began focusing heavily on apps and e-readers as platforms for its document sharing. The company also has evolved its e-commerce with its two-year-old Scribd Store, which is an attempt to bring together both mainstream publishing houses like Simon & Schuster (NYSE: CBS) as well as independent authors looking for large venue to market their own works.

There was a previous Scribd mobile app that was focused on accessing documents via mobile, but the idea behind Float is about arranging editorial content on on any device. Instead of just adding on to the existing Scribd-branded mobile product, the new app was named Float because “we wanted to convey the feeling of no boundaries; removing the barriers of time, space, sharing, devices. It also represents the fluidity of text,” CEO/co-founder Trip Adler said in an e-mail.

“Digital reading is fragmented,” he added in a statement. “There are different formats, devices, applications and paywalls for reading web content and we want to offer a better solution.”

While it sounds a lot like a hybrid of the long-form focused Instapaper or the more snackable news reader Pulse, Float has a few things which it believes can differentiate it. Aside from looking to develop stronger ties to online publishers, the app is said to be designed to work well for casual browsing as well as long-form reads. Text previously formatted for the web can now be easily read on any mobile device in a “floating text” reading experience, enabling users to zoom in on text and scroll left to right like the pages of a book or magazine. In addition, content is personalized to reading style and environment, including settings for direct sunlight and eye strain.

More than challenging Instapaper and Pulse, Scribd is, in a way, targeting Apple’s model, even while it looks to gain traction for its app on the iPhone (a Google (NSDQ: GOOG) Android version is likely to follow soon). Last summer, Scribd offered up its services to publishers who wanted a simple, PDF version of their magazines that could be easily read on the iPad using HTML5, as the Apple (NSDQ: AAPL) device didn’t recognize Adobe’s Flash software for viewing images. Scribd didn’t charge for the service, it only took a small share of the ad revenues for titles using its platform.

While major publishers have continued to spend lavishly in terms of money and time on creating app versions of their newspapers and magazines, there’s a realization that not all readers are equal. As they market subscriptions of digital versions of their magazines for devoted readers, part of the pitch of a platform like Float is that they’ll be able to attract new readers as they browse through numerous publications.

Although Scribd did not say anything about whether there would be an ad-sharing deal involved with titles featured on Float, at least at this point, the idea is that the app will serve as a discovery vehicle for new readers (naturally, the app contains sharing buttons for Facebook and Twitter). And as publishers have found, given the wide range of options web users have to access content, they need to be wherever potential readers are.

Scribd already has some premium content available, but it wants to be more diversified than its current products. For example, users now have the choice to purchase downloads of archived content — specifically, content that’s more than several months old. It also has been adding material to its Scribd Store as well over the past few months and is still looking to strike agreements with publishers.

The new products will likely involve more ways to access certain kinds of archived material. Last fall, the company introduced a co-branded reader that was more focused on driving ad sales. The company is still working on setting up an ad share program with publishers in the near future.So far, it has over 150 publishers — and another 100 media companies — using the site and Scribd believes there’s a number of other avenues it can mine.

Forbes is the first major mag publisher to use the Scribd platform to create a digital magazine replica. In a conversation with Jared Friedman, CTO and co-founder of Scribd, told paidContent that since the free, online-only Forbes “special issue” on Warren Buffett was released on May 25th, it’s been read on Scribd’s site over 25,000 times. A print version of Vintage Warren: The Best of Forbes on Buffett, a compendium of past articles on the “Oracle of Omaha,” will hit newsstands next month.

Although better known as a free document sharing platform, Scribd has been working for the past year to build up its revenues and capitalize on the growing interest in e-books and now, magazine apps. It’s been about a year since it opened the Scribd Store, which marked a sharp change in its strategy to drive revenues. While the Forbes issue is not technically a magazine app, it certainly can act the way apps like Wired’s does, including allowing interactive content and ads. Because it’s HTML5, the downloaded Scribd digital mag would have enough cached material so that users can still read even if they’re not connected to the internet.

Scribd is currently setting up trials with several other magazine and book publishers, but the company wouldn’t identify who they are. Aside from the low-cost of simply converting content to HTML5, Scribd’s self-service functions also obviate the need for extensive contracts with publishers.

The last part of the pitch Scribd is making to publishers involves the promise of a better user experience. “Publishers are a little lost and don

The effort is part of an “open strategy” that will make Scribd’s documents compatible with all manner of e-reader devices, including the iPhone, iPad, Amazon’s Kindle, Google’s Android system and beyond. Scribd, which wants to build on the roughly 50 million users it attracts every month, realizes it’s going up against the forces of DRM, although the EPub open format has been getting some traction.

Still, Scribd will have to convince mobile users that the trade-off for free and open document sharing requires putting up with ads around some content and paywalls on copyrighted works from its e-commerce store, which offers publishers an 80 percent take of the revenue.

E-commerce has been an area Scribd has wanted to grow. And while it sees an opportunity with the proliferation of devices, the e-commerce effort has remained fairly small since it started last spring. Currently, the WSJ notes, only 5 percent of the e-books on Scribd are copyrighted. For now, with devices fighting over prices for copyrighted material, Scribd is content to make its big mobile push with free stuff. After it hooks more users with its free docs and e-books, the hope is that it could make some more moves on the e-commerce end.

Compete stats show that Scribd trumps Docstoc in terms of overall traffic, but Docstoc has upped the ante with the launch of DocStore, an online marketplace where people can buy and sell their documents. Previously, Docstoc only made money by running ads alongside the over 3 million publicly-available documents it had aggregated; this follows the launch of the Scribd store in May.

The DocStore is initially launching with 12 content partners, including Legal Zoom and Podium Notes; the roster seems more oriented to small business owners and startups than Scribd’s. Other companies or users can apply to have their content added to the store; it runs on a rev-share basis. CEO Jason Nazar told me that the split favors the content owners, and that Docstoc provides real-time sales analytics, as well as some copyright protection.

Nazar said selling documents had always been part of the business plan — the startup just needed to scale out first. (Internal stats peg the company’s monthly uniques at 8 million). And while he expects a majority of Docstoc’s revenues to come from ad sales for the rest of this year, Nazar said the plan is for the e-commerce business to “take over” in 2010. Given the increased adoption of e-readers and e-books, there’s a very healthy market for both Docstoc and Scribd’s stores — though Docstoc could fare better if it continues to go after the smaller, indie market, as opposed to trying to compete directly with Scribd.

The company also hired George Consagra, most recently COO of AOL-owned Bebo, as President. He will oversee the expansion of Scribd’s business operations, including biz dev, sales, finance and marketing.