Along with "The Hero With a Thousand Faces," this is Campbell's greatest work. Campbell was a loving student of Native American cultures, and this book's historical achievement is to evaluate and compare all world mythologies as co-equal, including cogent and detailed examples from Native American mythology.
Campbell's core belief was that all humanity has a common origin, and that the study of mythology exposes this core identity amongst all peoples. By traversing the plains of time back to the very first artifacts of human behavior, he draws a compelling conclusion that we are all born of the same stock, from the same mythopoetic and spiritual origin, and destined to share the same future.
The student of humanity will find this study particularly compelling because Campbell identifies several mythological themes that span the globe. Among them are the virgin birth of a savior, the trial of the hero at the hands of evildoers, and the resurrection of the savior/hero from the dead. To my mind, these timeless echos of Christian beliefs place Western thought in an ancient and endlessly rewarding intellecutal context.
Campbell's higher purpose of showing that all humanity is united through its most fundamental ideas about the cosmos and our place in it is brilliantly synthesized in his discussion of the origin of agrigculture at the outset of the Neolithic. In the same way that all philosopy is "footnotes to Plato," all of history is "footnotes" to the Neolithic Revoltuion. Campbell handles this insight with a genius that must be read and re-read to truly appreciate.