Where’s the digital library?

I consume a fair bit of media, I’ll watch a decent amount of television, a bunch of movies, I listen to a lot of music and I read a lot of books. More and more my consumption is moving to be exclusively digital. Sure we’ll watch a fair bit of TV live over our cable, but I’m also watching a lot of on demand (TIVO/ Sky+ call it what you will). For movies the UK doesn’t have Netflix but we do have lovefilm and for music between Spotify and iTunes I’m fairly satisfied. However when it comes to books I think a lot more can be done digitally.

I still buy the occasional physical book, usually ones that I want to keep for posterity on our bookshelf, but more and more I’m reading books on the iPad/iPod/Blackberry through the Amazon Kindle applications. But books are probably the only medium that hasn’t moved to a digital one-use service.

For Television shows I can catch up online, for movies there are services where I can send the movie back, but no such service exists for books. Why hasn’t anyone launched a digital library, where you can rent books and perhaps either pay a small fee for a short term download or pay a per use fee that’s a fraction of the cost of the book, similar to the rental option on iTunes?

Am I missing something or is this an opportunity that is waiting to be developed?