Michael Pollan of all people has a new book about the history and potential current uses of psychedelics in America. In How to Change Your Mind, Pollan reviews the scientific research around the use of these substances (focusing mostly on LSD and psilocybin) and even documents his own "trips."

Aldous Huxely, The Doors of Perception, The Perennial Philosophy, "mystical experience," scientific understandings, the mind/brain problem, William James – it's all here. And from a guy who mostly writes about food!

Coincidentally, the Buddhist Geeks have recently been doing a series on the use of various substances to induce "mystical experiences."

I don't know that I would ever go down that route (and my hunch is that many of the historical contemplatives I study also wouldn't suggest it, likely seeing it as a "craving for experience" instead of a cultivation of detachment and a simple love of God), but the topic is one of the most interesting in modern discussions of spirituality.