What are the limits of Kindle Unlimited?

Amazon's new e-book subscription service Kindle Unlimited may sound like a great deal for bookworms who want to read an unlimited number of books from the retailer's vast collection. But consumers should consider the program's limitations before committing to the monthly service fee.

The company launched the program Friday, boasting 600,000 titles for $9.99 a month. Surprisingly, Amazon chose to start a service that competes with its own Prime program, which offers access to the Kindle Owners' Lending Library as one of its perks, in addition to free two-day shipping. Though the company is now the biggest online retailer, selling an enormous selection of goods, CEO Jeff Bezos originally built the empire off books, and content remains an important focus for him.

The Kindle Lending library, which debuted in late 2011, didn't cause quite the stir that Kindle Unlimited has, because of an initially small selection of e-books and a lack of competitors. But Amazon seems to have launched its unlimited service in direct response to the growing popularity of subscription media services.

The company didn't say why it was launching Kindle Unlimited, but Russ Grandinetti, Kindle senior vice president, did emphasize that the program is the "most cost-effective way" to access both audiobooks and e-books through one service, thanks to Amazon's Audible audiobooks integration.

So how great a deal is this service? Here are five things to consider before signing up:

1. Amazon isn't going to have all the hottest titles in stock. The company has made 600,000 titles available, including some popular series like "The Lord of the Rings," "Harry Potter," and "The Hunger Games." It's also featured some new titles, including "Flash Boys: A Wall Street Revolt" and "Capital in the Twenty-First Century," both released earlier this year.

But if you want to read novels that are currently at the top of The New York Times best-seller list, you won't get them with Kindle Unlimited. These include books like "Act of War," "Unbroken," and many other titles from major publishers like Simon & Schuster, Penguin Random House, and Hachette. In fact, it seems Amazon's new library lacks any books from the top five publishers in the industry.

2. If you're a Prime member, you already have access to the Kindle Owners' Lending Library. The library now has more than 600,000 titles as well, but it's not the same catalog, according to Amazon. That means there are titles available with the lending library that aren't available with Kindle Unlimited, and vice versa. Also, the Prime program allows users to borrow only one book per month, and doesn't include the complimentary Audible service.

Additionally, you can access Kindle Unlimited from any Kindle app, while the Prime lending library works only on Amazon's devices. Prime costs $99 a year (and includes many other services, like free shipping and streaming movies) while Kindle Unlimited's pricing makes it $120 a year.

3. There are alternatives. There are other platforms that were already running subscription services, including Oyster and Scribd, which both have deals with at least two major publishers: Simon & Schuster and HarperCollins. Oyster, which launched in late 2013, gives its members access to 500,000 books for $9.95 a month. The platform is compatible with the Nook HD, the Kindle Fire, and most iOS and Android devices. Scribd, has 400,000 titles for $8.99 a month and works across any device.

4. But what about the audiobooks? Amazon does have an advantage when it comes to audiobook fans. Its new subscription service features unlimited listening for 2,000 audiobooks. Powered by Audible, which Amazon bought in 2008, the feature syncs audiobooks with your e-books. Amazon is also adding a 3-month trial Audible membership with a Kindle Unlimited signup. Audible's membership starts at $14.95 per month and comes with a library of 150,000 titles.

5. Kindle Unlimited isn't doing indie authors any favors. Just as streaming services such as Spotify have changed how musicians make money, e-book subscription services like Amazon's could make it tougher for independent authors. One of Amazon's self-publishing platforms restricts writers from publishing their work anywhere else and allows Amazon to automatically include those works in Kindle Unlimited, which potentially takes away sales the book might otherwise have had on the site.