While I don't like the idea of forcing an author to open source their books, I really do dig having digital copies available when they are offered.
I have almost every CD Bookshelf O'Reilly ever published. I even owned most of the books in them (well mostly the Perl and Web stuff). It is handy to have searchable text for a good chunk of my Perl library no matter where I roam, online or not.
That's all I would like to say. Digital copies of books rock.