Tome of Magic
Tome of Magic is the name of two Dungeons & Dragons splatbooks; the first published for Advanced Dungeons & Dragons 2nd edition by TSR, the second published by Wizards of the Coast for Dungeons & Dragons 3rd Edition. Both share the common theme of seeking to expand upon the presence of magic in the D&D game, but they do so in different ways.
In Advanced Dungeons & Dragons, the Tome of Magic is a setting-neutral sourcebook for Wizards and Clerics - the Forgotten Realms had multiple splatbooks that served a similar role; Pages from the Mages, Wizards and Rogues of the Realms, Warriors and Priests of the Realms, and Secrets of the Magister. It provided an assortment of new spells and magical items for both classes, as well as three large indexes of spells covering all of the splatbooks to that point, but also introduced several new rules.
For clerics, those new rules consisted of three new spellcasting systems - Quest Spells, Faith Magic, and Cooperative Magic, as well as an assortment of new Priest Spheres: Chaos, Law, Numbers, Thought, Time, Travelers, War and Wards.
In Dungeons & Dragons 3rd Edition, the Tome of Magic was a way to introduce three new alternative types of magic; completely new systems, each with its own signature class, as well as appropriate new Prestige Classes, magical items, monsters and sample organizations.
The first of these three new systems was Pact Magic, exemplified by the Binder class. This is generally considered the best of the systems added, being unique, flavorful, relatively easy to understand, and with solid mechanics.
The last new system was Truename Magic, exemplified by the Truenamer. Sadly, this is considered the absolute worst of the systems introduced in this book, being considered underpowered and overly complex.