FWIW, if you haunt the Humble Book Bundle, you will end up acquiring a significant percentage of O'Reilly's library over time. O'Reilly itself does bundles there fairly often, and O'Reilly imprints like Make: and No Starch have put all but a few books that they've published in the last several years up in humble bundles.

One of the last bastions of good DRM-free eBooks is Pearson/InformIT, which includes a lot of big ones like Cisco Press, Microsoft Press, Addison-Wesley, Que, Sams, etc.