The Artist's Way series by Julia Cameron. I return to these again and again. A 12-step recovery program for the creative soul.
The Book of Learning & Forgetting by Frank Smith. A gentle and probing look at how people really learn, and how the so-called experts can get it so wrong.
Straw Dogs by John Gray. Not the Sam Peckinpah film, and not the Venus & Mars author. Unblinkered essays on human culture and civilization.
The Culture of Make Believe and A Language Older Than Words by Derrick Jensen. Heartbreaking and raw, but worth every moment spent with them. My copy of Language is so well-thumbed it has fallen apart (no small thanks to the modern binding process, which is so inferior to past practices it has virtually guaranteed that none of the paperbound books produced in the last 30 years will outlast the current generation of readers. A pathetic state of affairs.)