It's worth getting a book which explains the lessons so that you can read up in advance.
Its in fact essential to get and read a book on the PPL course. You should read the long briefing for each lesson before the air exercise, indeed a good instructor would have already steered you in this direction and prompted you to read the next chapter before your lesson. The majority of actual learning takes place away from the aircraft, use this 'away' time wisely and you will not waste your money in the air plus you will become a better pilot.