Watch out, Oyster and Scribd: Amazon's wading into the ebook subscription waters.

The e-commerce giant recently launched test pages for "Kindle Unlimited," a subscription service that offers "unlimited access to over 600,000 titles and thousands of audiobooks on any device for just $9.99 a month." The landing page was quickly yanked, but not before they were noticed by Kindle Boards users and reported on by Gigaom.

A page dubbed "KU Test" is still live and showing 638,434 results as of this writing, though the individual listings still show the full Kindle purchase price. Most are pretty meh, though you'll find some high-profile gems sprinkled around, like The Hunger Games, The Hobbit, Water for Elephants, The 7 Habits of Highly Effective People, Slaughterhouse-Five, 1984, and the Harry Potter series. Gigaom keenly points out that most of those bigger-name titles are already available through the Kindle Owners Lending Library, which lets Amazon Prime members check out one free book per month.

Netflix-for-books-style subscriptions are nothing new, but Amazon's the proverbial 800-pound gorilla in the digital book marketplace and the mere hint of it rolling out an all-you-can-eat option must have Oyster and Scribd mighty worried indeed--especially considering Amazon's mammoth user base.

But a subscription service is worthless if it doesn't carry stuff you actually want. While their digital shelves technically hold fewer ebooks than Amazon's if the KU Test's 638,434 count is accurate, Oyster and Scribd have already signed deals with several publishers, including HarperCollins and Simon & Shuster. Those deals have brought books from Stephen King, Ernest Hemingway, Mary Higgins Clark, Ray Bradbury, Walter Isaacson, Dan Brown, Janet Evanovich, Neil Gaiman, and more to those services--star power that Amazon's intial all-you-can-eat ebook lineup doesn't appear to match...yet. Amazon could likely close that gap quickly--if it stops pissing off book publishers, that is.