The Sound Reinforcement Handbook is the single best reference for audio engineers anywhere. Read this book and you'll know more than 80% of the guys calling themselves "sound engineers" out there. If you can do math, 90%.
Blues Guitar Inside And Out: I gave this to my brother when he started playing blues guitar, and now he's a great blues guitarist. The book obviously gets the credit. Plus, I managed to cite it in a constitutional law article once.