No harm in getting ahead of the game in reading material before you start a course. However, I would recommend you study the PPL groundschool first, if you are going to do a PPL. If you are going to an ab-initio course, then you could read - Mechanics of Flight by A. Kermode. Also a meteorology book for pilots, and maybe The Naked Pilot (I forget the authors name). These are easy reads. Some ATPL theory can be quite dry, and it's hard to get through, particularly if you are doing it in your spare time, so pick some well written books which will make it a bit more fun.
Of course, you already have a class 1 medical?