My publisher O'Reilly Media just rolled out a nifty feature, letting you sync your O'Reilly ebooks with Dropbox. Buy a book, and it pops up in your Dropbox automagically. You can choose which of the many available formats to sync (ePub, PDF, Kindle, DAISY, or Android), all of them DRM-free.
And hey, your first test run might as well be with my book, Tapworthy: Designing Great iPhone Apps, if I may be so bold. If you have the print edition but not the ebook version, you can snap up the ebook for just $4.99. Just register the print book with O'Reilly, and you'll see the option to "upgrade" to ebook on your products page.