Real Magic by Isaac Bonewitz

Every science fiction and fanstasy writer should read this book. Bonewitz has the first "Bachelor in Thaumaturgy" from a Californian University. He takes the assumption that psychic powers exist, and then runs with that through a very logical and rational process to how they must work in a universe where such things are true, drawing on anthropology and psychology and the occult. A facsinating read for believers and skeptics alike. Invaluable for writers. If psychic powers are real, this is how they would work.