O'Reilly really is one of the most enlightened publishers going. If you are interested in technical reference books, they have a two-for-one sale going on through April 30. Enter code "BYEBK" in the checkout cart.
As they say, "Download in 4 DRM-free formats:
PDF, .epub, Kindle-compatible .mobi, and Android .apk".
Their web site is:
http://oreilly.com/ebooks/
Any publisher that makes a point of selling DRM-free books gets my cash.
Robert