I understand there are 7 in total - one for each exam
There are 7 in total... but not one for each exam, no.
Book one is not for an exam, it covers the flying syllabus - the exercises you will fly and the briefing which should preceed each exercise. Book 5 is for the IMC rating, and is not relevant for a PPL student. A couple of the books cover two exams, though, which is how you get all the exams into less than 7 books.
Book 7, which covers the R/T, is a waste of money. Instead, buy CAP413 - in fact, you can download this free from the CAA's website and print it if you don't fancy buying it (sorry, don't have the link to hand). This covers the same stuff, but is more definitive (there are a couple of areas where it differs slightly from Thom).
So, if you're going for Thom, you should be looking to buy books 1, 2, 3, 4 and 6.
FFF
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