If you have a Amazon tablet, such as the Kindle Fire HDX 8.9, or the Fire Phone, you can use Oyster or Scribd. If you use a Kindle e-reader, Kindle Unlimited is the your best bet here, since you cannot download Scribd or Oyster books on the e-ink readers. The upside, is that unlike Oyster or Scribd, you can also shop for paid e-books on Amazon while also grabbing free books. One of the biggest downsides of this service is that browsing and searching Amazon's website for books can be tedious, since even when you search within the Unlimited titles, you'll still get results that aren't free. You can also use the Kindle apps for iOS, Android and Windows Phone to find and read books. There's no dedicated Unlimited app instead, you sign up for the service, browse Amazon's website for books, and download them to your Kindle device. There are also plenty of classics, such as 'Animal Farm,' 'Moby Dick' and Mark Twain's 'The Adventures of Tom Sawyer.' The book catalog includes many popular and best-selling books, some of which you won't find in the other services, including the 'Hunger Games' and 'Harry Potter' series. Unlimited costs $9.99 per month, and offers around 600,000 books that you can rent for as long as you want.

But like a library, you do not own the books you read, and if you cancel your subscription, you can no longer access any of the titles you've saved. Unlike a traditional library, there are no due dates, so you can hang on to a book for as long as like, just like you would with a Netflix DVD. You can read as many books as you want, for as long as you want, and with each service, you can download books for offline reading.

With each of these e-book subscriptions, you get unlimited access to their entire book libraries for a monthly fee.

Kindle unlimited free audiobooks