ONTRACK, as mentioned by Whopity, I would add initially PART1, PART2 and PART4 (once you grow as FI you can invest in the other ones).
The school I did my FIC had those books and your school may provide you with their own standardized material.
I bought
before the FIC, it was a nice way to calm the curiosity of what to say and how to say it during the first hours as FI(R).
Flight Instructors Manual from FAA and others CAAs like CASA and NZ (careful with the difference on EASA lessons, it is a nice reading in regards to the teaching part).
The PPL Volume1 course books like POOLEYS or AFE, containing all the exercises for the PPL it was very helpful for me to keep refreshing my memory (my PPL was done in 1997!).
POOLEYS PPL Exercises
AFE PPL Exercises
Once you are in the course or once you finish, do your own Presentation *PPT or PDF to keep them handy for your future lessons or briefings, why your own presentation? because when you build them you are also reinforcing your knowledge and self confident in it.
Once you finish the FIC you will keep learning from the instructing until well above 1000 hours!
As a personal point of view, it is a very rewarding activity to share what you like and to see your student growing and progressing in Aviation.
Good luck.