I used Thom as well, but with hindsight I'd forget the book and do Human Factors from the Confuser. I know you shouldn't really do that - you definitely shouldn't with any of the other exams - but the PPL level exam covers so little material that spending £18 on Thom's book is IMHO a waste of money.
At ATPL level all the questions are based on material in "Human Factors for Pilots" - or so my instructor says - so if you're heading in that direction Green et al. is supposedly the one to go for.