How Amazon's Kindle Unlimited e-book service compares with rivals Scribd, Oyster

Amazon is the latest — and largest — company to offer unlimited e-books for a monthly fee. Here's how Kindle Unlimited, which Amazon announced Friday, compares with rivals Scribd and Oyster.

PRICE: Kindle costs $9.99 a month, while Oyster costs $9.95 and Scribd $8.99. All three offer the first month free.

SELECTION: Kindle has the largest number of titles, with more than 600,000, compared with Oyster's 500,000 and Scribd's 400,000. However, only Oyster and Scribd have books from two of the largest publishers, HarperCollins and Simon & Schuster.

LIMITS: You can read as many books as you want, but you can have only a certain number of titles downloaded at a time. For Kindle, it's 10 per account. You have to "return" a book to get the 11th. Oyster allows 10 per device and Scribd allows 20 per device. With all three, you can read a book on up to six different devices.

DEVICES: All three work on iPhones, iPads and Android devices. Scribd and Kindle also work on personal computers and Windows devices. Only Kindle supports webOS and BlackBerry and dedicated Kindle e-readers such as the Paperwhite.

EXTRAS: Kindle is alone in offering free audiobooks as part of the subscription, with a selection of 2,000 from Amazon's Audible business. Subscribers also get a three-month trial to Audible, which offers one book a month from a larger library.