Funny coincidence, I just found this video http://www.reddit.com/r/haskell/comments/1l4ph3/dsls_and_tow... back, where the speaker mentions many CT ideas (adjoint functors, galois connections) and end up pointing at abstract interpretation and Lawvere work.

Seconded. To elaborate, it is essentially a lightly edited transcription of all the lectures of an undergraduate introduction to categories course, complete with diagrams, exercises, and dialogue, and even including students' answers (right and wrong!) to review questions. It's a very unintimidating format, and the book does very well to help a beginner get firm footing with the foundations.