I agree about DRM on books being bad. That's what I love about Pragmatic Programmers' e-books, I can get a plain PDF that I can read on any device that can read it.
The best format, IMO, is plain HTML. It can come with a custom style sheet, or can be overridden to taste on the client. It can be read by anything, it can be endlessly adapted .. it's just the best overall solution, IMO.