The Kindle and other e-book formats are very locked-down. They can be accessed through special apps, but they can't be read by other programs. You can't back up your kindle books on your PC hard drive. The things you can do to manipulate a commercial e-book file are very limited.
Some people think that e-books will eventually go DRM free, because Apple used to use DRM on iTunes, and then stopped. I think this is unlikely.
Music publishers went DRM free because they really didn't have a choice. CD-audio is an unprotected digital format that can be ripped DRM free to a sharable file by anyone. And publishers can't put DRM on CDs, because the media must be compatible with existing CD players.
If commercial music downloads are encumbered by DRM, they're inferior to freely-available pirated versions. E-vendors had to go DRM free because they were essentially competing with illegal downloads. People who might have bought the album would pirate to avoid the limitations on the commercial files.
A book is different because it can't be ripped like a CD. Somebody has to scan every page of a book to PDF or type every word into a text document in order to turn it into a file that can be shared on the internet. This doesn't happen very often, and the result is noticeably inferior to the original. Book piracy exists, but it's happening on a much smaller scale than music piracy.
I don't know anyone who has ever read a pirate e-book. Almost everyone seems to have downloaded music illegally. If publishers start releasing books as freely-sharable files, they'll create mass book piracy where none exists now.
A full book in Word format is less than 1MB. If publishers put their books out as DRM-free files, a reader could attach an author's entire life's work to an e-mail and forward it to everyone on his contact list with a push of a button, and there's nothing anyone could do about it. Sales would collapse almost overnight.
Nobody who is in the business of selling content would willfully create that kind of situation.
DRM free e-books will never happen; it would be the end of commercial publishing.