You might get eye strain after reading a few chapters, but that hasn’t stopped both Google (NSDQ: GOOG) and Amazon (NSDQ: AMZN) from making more e-books available on mobile phones. On Thursday, Google said on its Book Search blog that the 1.5 million public domain books it had scanned, and can be accessed for free on PCs, were now available on cell phones, including the iPhone and T-Mobile G1. In addition, the nytimes.com is reporting that Amazon is working on making the titles currently available on its e-book reader, the Kindle, accessible on phones. It did not, however, give a launch date or specify which handsets they would be available on.

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The two new efforts should be an interesting test on how long tail content fares on the mobile. Though Google has now dramatically increased the number of e-books available on a mobile device, the titles are public domain books, meaning that they are not the latest releases, and may not prove as popular. Kindle, which currently has an inventory of 230,000 titles, offers the newest releases, including many on the top-seller lists.

Interestingly, it wasn’t an easy job for Google to get the scanned books onto mobile devices, which the search giant itself called a “difficult engineering task.” Unlike Google Book Search for PCs, which simply shows the scanned images of each book page, the mobile version had to strip the text out for faster downloading on phones. But extricating only the text proved a challenge for its algorithms, which sometimes returned gibberish when it was presented with old fonts, or fancy writing, or even torn pages.

Can mobile phones challenge dedicated e-readers, and perhaps even replace them? There are already a number of apps out there that can change a phone into an e-reader, most notably Lexcycle’s Stanza and Fictionwise’s eReader. Analysts, however, think this scenario is unlikely. E-Book readers have certain technological advantages to mobile phones, including bigger screens, better lighting, and longer battery life. As the nytimes points out just as cameraphones didn’t replace digital cameras, so the ability to read books off your cellphone will probably not replace dedicated e-readers. Mobile phones will most likely provide a quick browse or limited read, as consumers trade off a mobile phone’s disadvantages for the convenience of reading on the go.

Google, meanwhile, told the nytimes it wanted to make more books available on mobile phones, including those that are no longer in print, and the books they currently scan with the permission of their publishers.