This book is an outline the Visuddhimagga which is considered the "great treatise" of Theravada Buddhism, an encyclopedic manual of doctrine and meditation written in the fifth century by the great Buddhist commentator, Buddhaghosa.
From the back cover:
Meditation is an essential part of the Buddhist way of life and Buddhist meditation practices cannot be explained apart from the fundamental doctrines of Buddhism. Buddhaghosa, the author of the Visuddhimagga (The Path of Purification), gave elaborate expositions of these fundamental doctrines. As such his work has served not only as a manual of meditation but also as the standard work on Theravada Buddhism as a whole. No other school of Buddhism has handed down to us a work of such importance. Therefore the Visuddhimagga occupies a unique position in the field of Buddhism in particular and the religious literature of the world in general.
What is to the advantage of the scholar is sometimes to the disadvantage of the layman. While going through the elaborate explanations of a certain topic in the Visuddhimagga, one is liable to lose its thread and become confused. This book gives a clear outline of the system and its essential points so that the details can be understood without much difficulty.