Humble Bundle sells groups of e-books at ridiculously low prices, DRM free. This month, the bundles are all Wiley titles, including three of my books: Applied Cryptography, Secrets and Lies, and Cryptography Engineering. $15 gets you everything, and they're all DRM-free.
Even better, a portion of the proceeds goes to the EFF. As a board member, I've seen the other side of this. It's significant money.