I feel like a used CFI. Does that count?

I don't think I actually bought any books for my CFI training other than the CFI PTS (which is also available online). Most of the books you need you either already have from previous training or you can get for free online like the PHAK and PTS for CFI, CPL and PPL. If you're going to pay for a course to study for the written I highly recommend Sheppard Air.