After hints of an unlimited e-book plan from Amazon popped up earlier this week, Amazon is making it official with Kindle Unlimited .

As the earlier leaks suggested, the service costs $10 per month and includes unlimited access to over 600,000 e-books, along with "thousands" of audiobooks. It's available through any device that runs Amazon's Kindle app, including iPhones, iPads, Android devices, and Windows Phones.

Although Amazon already offers a selection of e-books as part of an Amazon Prime subscription, that service (called Kindle Owners Lending Library) only works on Kindle devices, and only lets users check out one free book per month.

That's not to say Kindle Unlimited has no limitations of its own. While users can read as many books as they want, the selection doesn't include books from the five major publishers, who are already wary of Amazon and may be afraid of subscriptions undercutting full-price sales. (A couple of competing services,Oyster and Scribd, have the participation of HarperCollins and Simon & Schuster.) Amazon's service is also only available in the United States for now.