There's now a section on OReilly.com offering free ebooks about computer programming. There's four free Java ebooks and seven about Python, as well as an "Other" section which contains ebooks like C++ Today, Swift Pocket Reference, and Why Rust? But there's also some broader categories for Open Source and Software Architecture ebooks, as well as separate sections for their free ebooks about Data, Security, Web Development, and the Internet of Things.
http://www.oreilly.com/programming/free/
http://www.oreilly.com/iot/free/
http://www.oreilly.com/data/free/
http://www.oreilly.com/security/free/
http://www.oreilly.com/web-platform/free/
http://www.oreilly.com/webops-perf/free/