Godel, Escher, Bach by Douglas Hofstadter http://en.wikipedia.org/wiki/Gödel,_Escher,_Bach There's no way I can describe it, it has to be experienced. Talk about mind expanding.
For an esoteric read, check out Psychedelic Anthropology by Alan Coult. It might be hard to track down, but my university's library had a copy. He spends a lot of time discussing obscure spirituality and berating the current state of academe. The best parts of the book are those in which he discusses humanity's lockstep march toward a hive mind.
Castaneda - great suggestion! I'd recommend the Tibetan Book of the Dead (the Evans-Wentz version with commentary by Carl Jung)
If you really enjoy philosophy and all of its relating subjects check out the Oxford Companion to Philosophy. It's a rather large book, but WOW....it's got so much great content, I've been reading through it on and off for about 2 months or so whenever I have it near by....amazing compilation of definitions, biographies etc.... http://www.amazon.com/Oxford-Companion-Philosophy-Ted-Honderich/dp/0198661320