I've studied for both exams and passed both exclusively by using this book and the Oxford PPL test question CD. The first 3 sections are for the longer General Principles exam; the last section of the book covers Performance and Planning.
This is by no means obvious (so you aren't asking a daft question). I had read and studied the whole book for the first exam without realising I'd done the material for the second one. A quick bit of revision and passed that too.
Although there's no defined order to do the exams in (apart from Air Law being mandatory before going solo), you might want to consider doing Navigation before P&P which is an essential addendum to any nav preparation.
I've only used the Trevor Thom books myself and found them extremely well written and easy to study.
Good luck.
