Start with your FOI (Fundamentals of Instruction) which in FAA parlance is the Aviation Instructors Handbook.
Make sure you get the proper PTS (there are 4 for the CFI series, Single engine, Multi engine, Rotor and Instrument). If you are just studying for a CFI-A, get the single engine.
Finally, I like William Kershner and have his entire series of books. He has an Instructors Handbook. You have to be a little math oriented, a little twisted and you need to study from some other source, but he captures subjects that nobody else will touch.
Get with your CFI (if he is eligible for CFI signoffs - read the rulebook) and just learn to fly right seat. After you are comfortable and can land from over there, start doing all the PTS manuevers. You will be "ham-handed" for about two flights and then you won't care that your left hand is on the throttle, the right hand moves the yoke and the instrument panel is about two miles to your left.
Good luck.