When I did my FIR it we covered everything relevant to PMI as part of the ground training, a mixture of the Aviation Instructor's Handbook (some) and the full CAAP (most), so the only self study was a review of the class material - isn't this the case with where you're doing the rating?
I think I was part of the first wave of new FIR(3) training that needed to take PMI, and it wasn't that difficult TBH mainly rote learning of the required material. You can't go wrong by revising the full CAAP on the subject. I forget which one specifically though so I can't give you the ref.
Be sure you have your long briefs nailed for everything up to advanced stalling as these are the most theory dense briefs that test your knowledge which they will most likely go for in the test. Good luck!
