I'm no CFI as of yet, but I just got a handful of the FAA books that turned out to be really good. They're free online too, but I couldn't stare at my computer screen that long. I got the Weather Services manual, Handbook of Aeronautical Knowledge (seems like it might be useful for lesson plans), and the Instrument books.
William Kershner wrote a few books that I hear are really good. I found his flight instructor manual at half price books for 7 bucks, but it's like the 1983 version or something. I haven't had time to look through it really, but the guy was a classic aviator and I've never heard anything but great things about him.
The advisory circular "Aerodynamics for Naval Aviatiors" is pretty sweet. I'm not sure if it would help much for your CFI stuff, but it's pretty interesting and you can probably blow the examiner away with some of the material from that one.
I have another books called "The Illustrated Guide to Aerodynamics" by H.C. "Skip" Smith that is pretty solid, though not nearly as detailed as the Naval Aviators book.