I read a ton (voraciously is the word normally used), and I keep coming back to Thoreau's "Walden." Not only is Thoreau a decided naturist, but he's an insightful student of world religions and an astute observer of reality.
I read it again every year or so, and I'm blown away every time. It was also a surprising point of contact with my grandfather - I worked with him on the farm one summer, and his easy and peaceful attitude toward nature reminded me of Thoreau like crazy. I asked him if he had read Walden, and he said "Oh, sure. 3 or 4 times at least." I never thought of my grandpa as a reading man, being an old farmer from Amarillo, but the man had some deep insights that changed my perspective on him, Thoreau (my grandpa pronounces it "Thoroo," :), and the world. Great stuff.