George Van Eps - Harmonic Mechanisms

A great series of books by master guitarist and pioneer of the 7-string guitar George Van Eps are the three "Harmonic Mechanisms for Guitar" books.

There is a lifetime's worth of studying to be found concerning guitar technique, which is taught through a complete knowledge of the guitar fingerboard.

It is quite 'classical' in approach, with lots if attention to scale/arpeggio exercises. However, once studied, I think the material provides an astounding knowledge of the guitar that can easily be applied to any style of music, and probably even other instruments as well.