I'd suggest taking a look at
O'Reilly's website. They have a good reputation, and have books covering every aspect of computer science in both paper and electronic formats. They also do print + ebook deals, so you can save money if you want both paper and e-book. The e-books are available without DRM in various formats (most are available in PDF, ePub and Mobi)
You might also want to take a look at their
Open Books project, which is a collection of books with some form of "open" copyright.