The Illuminatus! Trilogy by Robert Anton Wilson and Robert Shea. Especially the appendices. It's lurid trash but it's also a delivery system for some very interesting ideas about thinking.
Understanding Comics, by Scott McCloud. Note: I spend a significant amount of my waking hours drawing comics. If you care in the least about comics, as a creator or a consumer, this book will give you a lot to chew on.
d'Aulaires' Book of Greek Myths, by Ingri and Edgar d'Aulaire. I credit this and their book on the Norse myths with my being what I call a "polyagnostic"; I knew from a very early age that there are religions that have come and gone, that their adherents believed as intensely as the ones we have now. With these as a foundation it was very easy to see Christian myths as, well, myths.
And some stuff I've simply enjoyed a lot:
The Stress Of Her Regard, by Tim Powers. Vampires, the tendency of Romantic poets to die of consumption, and a secret history of the world. I've read a lot of his stuff but I keep on coming back to this one every few years.
Against A Dark Background, by Iain M. Banks. As a SF writer he's most well-known for his "Culture" books; this one is outside that continuity. It's both comedic and tragic, as well as endlessly inventive.