I would suggest going for the number 1 and 2 books of the airpilots series, that way you are following the route you will be going in when you start your PPL. The flying training book will give you a heads up on the exercises you will be doing and the aviation law & meterology will give you a headstart on those first 2 exams.. if you choose to take met after aviation law and depending on how your FTO want you to go about your exams. I don't know if it's manditory, it was at my club, but I had to get my aviation law out of the way first before I solo'd. If you've already gone solo in a glider does that mean you have done aviation law?