Completely disagree on the "Headfirst books" comment. All the head first books I have read (Java, Design Patterns, Android, C, Ruby) have very high quality content. I have read great books on all these subjects along HFs. i.e. The Design Patterns Smalltalk Companion, K&R, etc. The HF books really help visualize and actually enjoy the subjects, they also have great exercises that help to cement the concepts one has just learned. If any of the HF authors read this, thanks for these books, they have help us a great deal.