Whilst I agree you should not try to learn everything before you arrive, it certainly doesn't hurt to start reading a little bit when you are bored.
I would suggest that Aviation Law is the subject that quite a few of us have found hardest to learn, so perhaps if you have access to Trevor Thom's Air law book for PPL students it might give you a running start.
However, you WILL be given everything you need at the college, and the exams are now multiple choice up to Area. This means you have to understand the subject rather than just learn parrot fashion, but most of us preferred this type of examination.
Anyway, I would say just do a little bit of background reading here and there, but mostly spend the time looking forward to the start!