This is (as far as I know) almost impossible for mainstream books. But for computer books it's not too bad. Both Pragmatic Programmers and O'Reilly sell DRM free ebooks. And I've recently started buying ebooks from informIT (mostly Addison-Wesley ones).
The other issue with ebooks is format. The Kindle currently only handles mobi and pdf. And pdf's are generally painful to read on ebook readers because they have a fixed, usually large (e.g. 8.5 x 11) page size, with big margins. Because of this I bought one of the bigger 9.7" Kindle DX's, and it helps, but it's still not great.
The Kindle helpfully trims white margins on pdf's, but the problem is that the ebooks informIT is providing are images of the paper books so they have stuff like page numbers and titles in the top and bottom margins which prevent trimming from working properly. And worse, the stuff in the margins is not symmetrical so odd pages end up trimmed differently than even pages which means the zoom level keeps changing. Not the end of the world, but annoying.
Both Pragmatic and O'Reilly provide ebooks in epub, mobi, and pdf. But informIT only provides epub and pdf. At first I made do with the pdf's but eventually I got fed up and started looking for alternatives.
I thought about looking for some tool to process the pdf's and crop the margins. But that's not the only problem. Kindle doesn't let you highlight sections of text in pdf's, only in mobi. And the pdf's I've had from informIT haven't had any hyperlinks (e.g. from the table of contents) although pdf's are able to do that.
I took a look at the epub's from informIT and they seemed better, at least they were hyperlinked. So I looked for tools to convert epub to mobi. Stanza was one that was suggested and I already had it installed so I gave it a try. I like Stanza but it didn't do a very good job of converting these epubs to mobi.
The other tool that was suggested was Calibre. A plus for Calibre is that it's an open source project. The user interface is a little confusing but it did a great job of converting epub to mobi, including the cover image and hyperlinks. And it even recognized when I plugged in my Kindle and let me copy the books over. I downloaded the epub versions of all my informIT books and converted them - a big improvement.
I wish I'd got fed up sooner! I could have avoided a lot of painful pdf reading.
Although I'm happy to be able to get Addison-Wesley ebooks from informIT, I do have a minor complaint - their sleazy marketing. I got an email offering me a limited time 50% off deal. Sounded good to me so I found a few books I'd been planning to get. They came to something like $80. When I entered the 50% off code the price only dropped to around $70. That's a very strange 50%. Looking closer I found that you normally get 30% off, and over $55 it goes up to 35%. So the 50% deal was only 15% better than usual. I realize this is a common marketing scam, but it still annoys me. O'Reilly plays some similar games but not quite as bad.