Tome of Arcane Brilliance

Tome of Arcane Brilliance teaches a mage the spell Arcane Brilliance (Rank 1). Learning the spell consumes the tome.

As of Patch 2.1.0 Arcane Brilliance (Rank 1) can also be learned from a mage trainer.

Source
This tome drops from elites in level 60 dungeons, such as Scholomance, Stratholme, Dire Maul, Blackrock Spire, and Blackrock Depths. It also drops from certain level 60 raid dragons, such as Onyxia and the Green dragons.