Tome of Arcane Brilliance 2

Tome of Arcane Brilliance 2 teaches a mage the spell Arcane Brilliance (Rank 2). Learning the spell consumes the tome.

As of Patch 3.0.2 Arcane Brilliance (Rank 2) can also be learned from a mage trainer.

Source
This tome drops from elites in most high-level Burning Crusade dungeons, such as Tempest Keep, Shattered Halls, parts of Auchindoun, The Steamvault, Caverns of Time, Karazhan and also in heroic dungeons.