You could start with the FAA Airplane Flying Handbook and Pilot's Handbook of Aeronautical knowledge, available free online. That might save the expense until you know which school and the materials they might require.
The chap who asked the question's location says he is in the South East - I assume he means England so it is not appropriate for him to use American documentation because there are many differences in the regulations, terminology and practices.
He would not go far wrong in getting a copy of the first in the Thom series "Pilot's Manual Volume 1 Flying Training – APM EASA Book" available from Pooleys, Flight Store, Transair and others. All seem to charge £23!
My experience is that it is also desirable to read books from more than one author covering the same subject areas (preferably by begging or borrowing from someone who has just passed the exams) - you get a broader perspective that way.