Watts' "The Way of Zen" is the only philosophy book I own
An ancient paperback, apparently read many times by its previous owner(s), and twice by me.
But to be honest, I have never paid much attention to philosophy in the formal sense. From outside, the endeavor seems to be to create the most elegant metaphor (elegant in the math sense), then explain it to death and beyond.
I prefer my metaphors contained in a story, and the ones I've adopted for my own use come mostly from fiction, leavened (heh heh) with some of the Jewish sages: Hillel, Maimonides, Einstein and Gell-Mann.
Here's my favorite Big Metaphor: Genesis 18:22-33. Abraham has the chutzpah to argue with God... and wins.