Amazon has just introduced a new service called Kindle Unlimited. For $9.99 a month you can have unlimited reading to over 600,000 books and about 8,000 audiobooks.
If you have Prime, this probably isn’t really worth it. Its roughly the same selection as the Kindle Lending Library, and it doesn’t include most latest releases from major publishing houses.
But on the other hand, with Kindle Unlimited you could use it on any device, not just Kindles. And you have access to a small selection of audiobooks. Kindle Unlimited also includes a 3 month trial to Audible, but you can get pretty good deals just signing up direct with Audible (I got 3 months at half price, but they also offer a free month instead).