John Denker's
See How It Flies is one of the best texts that bridges the gap between the trivia books designed to be accessible to every PPL and aerodynamics books designed for professional engineers. I think it does a fantastic job of capturing the essential physics, without getting too heavy, and without bending the truth for expediency. The best bit of all is that the book, complete with full text and graphics, is on the WWW.
If you want to get 'heavier', Barnes McCormick's 'Aerodynamics, Aeronautics and Flight Mechanics' is hard to beat.