Well, I was recommended to buy Mr R.D Campbell weighty tomb called 'Flight Instructors Manual'. Turned out to be a marvellous bookend that only cost £30. This book is far too wordy and seems to be written in an era when handlebar moustache's and pipe smoker of the year awards were all the rage.
Personally, I found that the PPL Flight Training book from your man J.Pratt is as good a guide to instructors as it is to students. There are good diagrams of each and every exercise with clear and concise text. It is written in a way that is easy to follow and presents the info in a Janet and John way that even us instructors can understand.