It’s a cactus that has mescaline in it and induces visions and healing experiences. In The Hummingbird’s Journey to God, Ross Heaven approaches the plant from a state of interest and inexperience. He reports on his own use of the cactus, and shares the words of shaman who have worked with it at length, and other explorers who have taken San Pedro. There is some history, some wider information about psychedelics, and on the whole it’s a very interesting read.