There are a number of books which I consider invaluable. They have helped me form my own teaching method. Whilst they may not be so useful to younger students, more advanced players should consider them essential reading.
Scott Tennant's Pumping Nylon is a guitar method for the more advanced student. Scott offers advice on left and right hand techniques and nail shaping. There are plenty of exercises to help you understand his thoughts.
The Classical Guitar Pedagogy provides even more detail. This book is designed for guitar tutors but again, it is very useful for advanced players to understand how they are playing.
Solo Guitar Playing - Book One by F. Noad provides great reading material for the beginner and intermediate player. The book offers so many exercises that your reading improves through playing. It is also an excellent and thorough tutor book. Some of his methods contradict my thoughts, but then no tutor book mirrors my thoughts on teaching. That is the reason for this website!