Paperback books are heavy, they're prone to damage and creasing, they can be tedious to transport, hard to read and difficult to hold especially on public transport. Books are heavy, and if you've ever had to move house and pack and move books then you would know how heavy they are in numbers.
Books to Ebooks would be a service that would allow you to trade in your old books for their eBook equivelent or if not available, any other eBook from the collection. There would be DRM rights issues to overcome with this problem, but wouldn't it be cool if you could recycle your books for eBooks?
Better yet; schools, libraries and charities of course still use the original paperback book, so books could be donated and or sold at a really cheap price to these institutions.
Licencing is a complicated issue when it comes to movies, music and books. Rights holders are still living in the past when it comes to relinquishing control. Even though you're trading a book for a digital version which should theoretically be the same thing, in the eyes of the law, it can mean something entirely different.
If you can get major book publishers and eBook distributors on board, this idea would be a lot easier to execute upon.