Tim O’Reilly discusses Reinventing the Book in the Age of the Web” — and the context is his (and Sarah Milstein’s) new book about Twitter.
Perhaps the biggest driver, though, was the need for speed. We couldn’t imagine writing a book about twitter that wouldn’t be immediately out of date, because there are so many new applications appearing daily, and the zeitgeist of twitter best practices is evolving equally quickly. So we needed a format that would be really easy to update. (Again, modular structure helps, since new pages can be inserted without any need to reflow the entire document.) We plan to update The Twitter Book with each new printing.
This is very much my strategy with this book/website, though the things that change rapidly will change mostly here on the site, not in the printed version.