I have a decent-sized collection of literature on the topic. Here are my personal favorites:
Food of the Gods by Terence McKenna
Breaking Open the Head by Daniel Pinchbeck
Psychedelic Shamanism by Jim DeKorne
Storming Heaven: LSD and the American Dream by Jay Stevens
Acid Dreams by Martin A. Lee & Bruce Schlain
Vine of the Soul by Richard Evans Schultes & Robert F. Raffauf
PiHKAL: A Chemical Love Story by Ann & Alexander Shulgin
TiHKAL: The Continuation by Ann & Alexander Shulgin
The first 4 are more on the topic on psychedelics as they relate to spirituality and humankind, sometimes even a little bit speculative. The rest are more like reference or textbooks, factual and informative with less speculation or spiritual component (although PiHKAL and TiHKAL both contain some great theories and insights from the brilliant mind of Alexander Shulgin).
I have a few novels relating to psychedelics and just altered states in general, but I haven't gotten around to reading them yet so I don't know whether to recommend them or not.
Enjoy your reading!