Not to be confused with Tome of Magic.
This D&D 3.5 supplement introduced three new forms of magic, with rules different from the normal 'vancian' magic system used by wizards, clerics, druids, and so forth in the normal system.
Pact magic - By making pacts with the vestiges, remenants of dead and forgotten gods, you gain long term mystical powers and traits.
Shadow magic - Drawing power from the plane of shadows, you gain a smaller number of Mysteries (spells) which are usable multiple times daily, as spell-like abilities rather than as spells.
Truename magic - By speaking words of power in the true tongue of creation, you alter the world around you. Magic preformed by making skill checks with a truespeak skill to cause effects.
Together with Tome of Battle: The Book of Nine Swords, this is considered by many to be an early alpha test of the systems for 4th edition D&D.