The Wizard's Grimoire Revised Edition (1998)

From the publisher's website:

Magic is the supreme power of Mythic Europe, and this tome of knowledge contains a whole host of new directions for magic. It features new templates for player character magi, essays that examine topics of concern to the wizardly class, rules for improving laboratories in a myriad of ways, sample magic items that demonstrate possibilities and save storyguide time, more than a hundred new spells for use in your Ars Magica saga, and much more.

As a revised edition, though, this supplement isn't just a reprint. In addition to updating the original Wizard's Grimoire for the Fourth Edition, this book includes a substantial portion of all new material. Completely new rules for faerie magic and a sizable expansion to the Fourth Edition's new book rules only scratch the surface. Truly, this is a must-have item for all players and storyguides who play Ars Magica Fourth Edition.