Castaneda first met Don Juan in the early 60's, before the hippy movement, before psychodelic drugs became popular. He was studying anthropology in Los Angeles, and Don Juan served as a field source for some fading knowledge of tribal and shamanistic rituals in Northern Mexico. Castaneda was specifically interested in peyote, a plant that gives its users hallicinations and mixes the senses in strange ways, and which LSD was meant to be a chemical reproduction of. Castaneda's first book presents a very detailed scholastic interpretation of his experiences. All books after the first simply focus on Castaneda's experiences with Don Juan.
Castaneda's drug experiences are different from other accounts I have read, because they are intimately tied with the Yaqui philosophy and mythology. The drugs only serve as a means to an end, not as the end in themselves. The first 2 books in the series describe Castaneda's drugs experiences with Don Juan, but from the 3rd book on, the drugs disappear forever and Carlos' experiences are actually more fantastic, more amazing, more unbelieveable as he slowly becomes a practicing sorceror, traveling to alternate dimensions and battling other sorcerors. Many of the books seem to reach a definitive conclusion, only to have Castaneda's perceived understanding of Don Juan's teachings completely destroyed in the next volume. Again these experiences do not in any way compare to magic and sorcery you might find in pagan, christian, or celtic mythology, nor does the "world-philosophy" of Don Juan resemble in any way the wholistic ideals of Eastern religions like Hinduism or Buddhism. How can you describe things most people have never seen before? Castaneda does a good job, considering the fantastic nature of some of his journeys. However, while the visions and magical feats are mesmerizing to read, I often found that Don Juan gave his most profound knowledge while simply talking to Castaneda.
Do to its subject matter, most Christians will find these books offensive, but I promise you that nothing in any of these volumes will turn you onto the path of Satan :) There is a certain participation required from the reader - Castaneda repeatedly affirms that everything he has ever written is absolutely true, but intelligent readers will be constantly wondering if such fantastic things could possibly exist. By turning off this little nagging "naysayer" in my own head and allowing myself to simply sink into the mood of the work and believe, I found that I was better able to intuitively understand the very abstract concepts that overwhelm you. However, because these accounts are SO unbelieveable, SO fantastic, SO unlike our everyday life here in the US, I found that I never confused my world with Castaneda's. It's like reading a book about walking on the Moon - you can understand and appreciate what it must have been like, but you don't start wearing a spacesuit to work!
Don Juan also uses "power plants" to facilitate Castaneda's awareness of the land beyond the mind. It would be foolish to write this off as purely drug use. Our "modern" society aside, these plants of the Earth were often used for spiritual realization. It isn't the end, but can open a door...
This book is best understood in context of the other books in the series which go into greater detail on the essential teaching.