From the publisher:
Magic pervades many worlds of the Basic Roleplaying game system, for in the game every adventurer – every character – has the capacity to manipulate invisible powers. But players will know much more about employing, measuring, and calculating magical power than any adventurer, including the mightiest mage or the most powerful shaman in the game.
This book explains the mechanics of three independent magic systems — spirit magic, divine magic and Wizardry — and details ritual magic, a system common to shamans, priests and wizardry. These approaches differ fundamentally only in the perceptions of the users. Which is the “true” way is for you to decide. By constructing parallel magic systems, all of the familiar traditional and literary magical stances and devices could be simulated by game mechanics.