One great book for aspiring CFI's is :
"The Flight Instructor Manual" by Mr. Kershner. Available for $20.00 +S/H in it's fouth edition. PM for details.
You will quickly notice that the PTS and some FAA Pubs are derived from his book. You will spend some time laughing, as he used to have a tremendous way of making some of these subjects nice to read and interesting.
Another one I just got:
Bridgette Doremire just came out with the "Flight Instructor Notebook". I believe, I own the second copy off the press. Available from Qref.
Fly your complete commercial from the right seat. Talk. If you have any chance, and a patient enough instructor, teach your instructor during maneuvers - and do the same with your DPE during your Commercial CR.
Have fun!