Most of the textbooks are only available for purchase, but a few are available to rent, too. Amazon and Barnes & Noble also rent digital textbooks, and when I compared prices across the stores, I found variability in both availability and how long the rental periods are.

Take the Psychology textbook by Daniel Schacter, for instance: At Google, it’s only available as a rental, for $65.99 for 180 days. The textbook isn’t available in a digital edition at Amazon at all, but students can rent a print copy for $25.23 per semester (a period ending January 4, 2013), with the option to extend the rental for $5 every 15 days. And at Barnes & Noble, the digital textbook rental is $1 cheaper than Google’s: $64.99 for 180 days.

Google also has fewer digital textbooks for rent than Kindle or Nook, but the selection should increase over time. Google says it’s working with the five largest textbook publishers – Cengage, Wiley, Pearson, McGraw-Hill and Macmillan.

The company, which uses open-source content to create college-level digital textbooks with material similar to the books students are assigned, says it reaches a million students a month and offers content for 21 subjects. On Tuesday, Boundless launched its first paid product: a $19.99 “textbook alternative” that doesn’t just provide content but gives students a structured process for learning it.

“Most students aren’t good [at studying],” said CEO and co-founder Ariel Diaz. “They do things that feel like studying, like going to the library and using a highlighter. But that might not be the most effective way to learn.”

To help students actually absorb the content, he said, Boundless’ textbooks draw from learning science and give students a guided path for learning. Much like a tutor walks her pupil through material, stopping along the way to prod him for recall or remind him of older lessons, Boundless measures when a student is likely to forget something and serves up a timely flashcard or quiz. The technique, known as spaced repetition, has been proven in studies to help commit content to memory, Diaz said.

The company, which also launched its first native apps for iOS on Tuesday, said it will continue to offer a less interactive digital textbook in each of its subject areas for free.

Given the size of the textbook market, which is estimated to be about $8 billion, it’s little wonder that everyone from tech giants like Apple and Google to startups Kno, Inkling and Benchprep want a piece of it. But Boundless’s interesting and cost-saving twist is that it gets its content from free Open Educational Resources (OER), which means it doesn’t have to share revenue with publishers or pay licensing fees.

Still, it faces a bit of an uphill battle proving to students who have likely never heard of the company or OER content that its digital textbooks can stack up against more expensive and static titles from well-known publishers. And it’s not alone in providing students “smart” textbooks. Publishers like Pearson and McGraw-Hill education, as well as startups like Benchprep and Kno, market digital textbooks that include enhancements like interactivity, personalization and student progress tracking for teachers.

In April of 2012, on same day it announced an $8 million funding round, Boundless revealed that it had been sued by Pearson, Cengage and Macmillan Higher Education.  The lawsuit alleged several violations, including copyright infringement, unfair competition and false advertising. While Diaz said he couldn’t comment on the lawsuit, he said Boundless is in the midst of mediation with the publishers.

The main point of the breakfast was two bigger announcements: The Chromecast, a $35 dongle that lets you stream internet video to your TV, and a new Nexus 7 tablet. Nonetheless, getting into the textbook market helps Google compete against Amazon, Barnes & Noble and Apple, all of whom have already entered the digital textbook space. In general, Google has struggled to gain ebook market share against those companies.

Google says it’s working with the five largest textbook publishers — Cengage, Wiley, Pearson, McGraw-Hill and Macmillan – and that students will be able to purchase digital textbooks or rent them for up to six months. It also said that prices would be “up to 80 percent” off print list prices, which is the same claim that Amazon makes on Kindle textbook rentals.

Google is already selling a few digital textbooks through the Play store, but they’re not from the big publishers and they are not available for rental.

Given the size of the K-12 textbook market — $8 billion, as my colleague Laura Owen pointed out in a post on digital textbooks earlier this year — the opportunity is certainly there. But, considering that the procurement processes are largely controlled by state governments and school districts (and can often involve a decent amount of bureaucracy and red tape), it can be a difficult field to break in to.

To get around that hurdle, Kno isn’t targeting the schools themselves, but the parents.

“While the schools figure out what they want to do with their strategy… we believe that we can make this available to parents,” said Osman Rashid, CEO and co-founder of Kno, Inc.

The company’s pitch is that, for $9.99 or less, parents can rent an interactive, searchable textbook for a year and, at the very least, save their kids from having to carry a 25-lb. backpack to and from school every day. The books, which span several subjects, such as chemistry, English and biology, include links to videos, audio files, images and 3D models. They also come with digital flash cards and quizzes, as well as a journal that stores all of a student’s notes and highlighted content, which they can review even after the yearly rental expires.

Students can also write on the books and type notes (although functionality is limited with the current tablet keyboards and finger-based writing options).  In the coming months, the company plans to roll out an analytics feature that tracks student engagement, which the company said can be a leading indicator of student mastery and performance.

In January, Apple announced its own entrance into the K-12 market with an iPad-based textbook initiative launched in partnership with HMH, Pearson and McGraw-Hill. And, given its longstanding relationship with the K-12 market and the speed with which schools around the country have adopted the iPad, the tech giant has a built-in advantage.

But, for his part, Rashid said: “Apple being in the market is awesome. Ever since they’ve entered, I’ve never had to explain what a digital textbook is. As a startup, I’m saving a lot of my marketing dollars.”

Aside from Kno’s price (which is lower than Apple’s price of $14.99 or less), he said that his company aims to offer a deeper educational experience, the ability to access the books on multiple platforms (not just the iPad, but the Web, Android and Windows 7) and a larger catalog of books.

Through its partnership with HMH, Rashid said they cover about 48 percent of the market with a K-12 curriculum based on the Common Core standards, which have been adopted by 45 states. He said they already partner with Pearson and McGraw-Hill for their college offerings and hope to bring them over to the K-12 market as well.

As for its penetration in the higher education market, Kno said it offers 250,000 textbooks and journals in partnership with nearly 60 publishers and reaches students at about 5,500 universities and colleges.