Bruce Lee's "Tao of Jeet Kune Do" was a very big inspiration for me - although it is written for martial arts, (which I'm sad to say I haven't studied), it offers a lot of thought provoking which makes you re-think your approach to situations. I've found these concepts and philosophies can be applied specifically to guitar, and have incorporated the ideas into my practicing.

I've also found reading Buddist philosophy to be very insightful, although it's all a matter of taste I suppose.

I'm with Cryptic and Arika, everything and anything can be an inspiration, and I know I've been able to gain a lot from reading Steve Vai's philosophies on guitar and life; but ultimately, like everything else, it's all about what fire's your imagination and makes you stop and think - that sort of "wow!" factor I guess.