What if you had to use a different music player for music you purchased from Amazon?Well, what if every time you bought a book it came as a separate stack of paper in a form factor and layout that was chosen for you and which you couldn't change?
Oh, wait, that was how all books worked for five hundred years.
Perhaps you are looking the gift horse in the mouth a bit too sharply?
Anyway, a big reason why the music industry worked differently is that computer wizards invented machines that were really good at taking a legally-purchased CD and turning it into iPod-compatible digital music in five minutes without human intervention. That's just not possible for books. (Though it might eventually be possible for digital books, DRM or no.)
I think it even might be legal, and it's certainly low-risk, for you to buy a book, slice off the binding, feed every page through OCR and put the result into any format that you like for your personal use... but it's so much work that you'd probably rather spend the five or ten bucks and cope with the different book-reading interfaces. (If you're interested, though, I hear the Fujitsu Scansnap is pretty nice.)
The other factor is piracy, of course. Perhaps the need to compete against pirated books will eventually drive the book industry to shed DRM. I do wonder, though. Counterintuitively, books really may be harder to pirate than digital music -- at least for now, and for my taste -- because the lowest-common-denominator format is plain text, and plain text can be painful to read. After spending five minutes reading a Project Gutenberg book on the iPhone the other day, and tripping over poor spacing, miserable hyphenation, and misplaced chapter breaks the whole time, I found myself craving the ability to spend three bucks on a version that had been laid out by a competent typographer.
What I hope is going to happen is that eleven different iPhone reader apps will come out, and compete, and the profit margins will flow to the one that lets artists and typographers lay out the books in a thoughtful manner, and then everyone will copy that one. But, even if I get my wish, we might have to suffer through years of garbage first, just as we did with CD-ROMs, or laser printers, or the early days of HTML when the blink tag and the animated GIF roamed far and wide.