Hugh McGuire argues that the future of books will be as ‘web objects’, or websites:

The secret among those who have poked around EPUB, the open specification for ebooks, is that an .epub file is really just a website, written in XHTML, with a few special characteristics, and wrapped up.

McGuire has been one of the most active (and influential!) supporters of books as linkable, open websites. His work at O’Reilly is worth a look.