What books would you recommend besides the obvious FAR/AIM, and test
study guides?

I put together an online resource for CFI materials, here

http://faculty.chicagobooth.edu/john..._Programs/CFI/

I especially recommend the "FAR notes." Here I tried to put in plain
English the FARs that we need to know and teach as glider pilots,
without all the category III and other stuff.

John Cochrane