I read through Martha Masters' book a few years ago and enjoyed it. It's all too brief, but a nice selection of not-so-common repertoire adds value. It's organized somewhat like a series of master classes (no pun intended).
Currently, I'm revisiting "The Natural Classical Guitar: The Principles of Effortless Playing" by Lee F. Ryan, which is a deeper read. The first third of the book reprises the physical fundamentals, from sitting to planting, with a strong emphasis on balance, relaxation, and natural movements. The rest of the book gets into refinement through technical details (nails, tone, vibrato, etc.), and then on to things like learning and practice strategies, visualization, performance.
Ryan attempts a holistic approach based on Taoist practice. I was skeptical about that (the approach, not Taoism), but I'm finding it quite appropriate to, and integrated with, the subject. Mostly, there's lots of good practical advice, so far.