Very, very good books.
Well structured folders, divided into the different topics, with formulae and key facts at the beginning.
Based on feedback questions from a range of schools... but the real point of getting his material is for the explanations.
Currently he has a folder with 1000 questions for each of the following:
Principles of Flight
Performance
Instruments
(I think that's all of them, but he brings out a new one every now and then)
I have all 3.
Would definately recommend.
IH