Tome of Retraining

The Tome of Retraining unlearns all of the Hero's spells, allowing the Hero to learn different skills.

In the RPG
This small book looks more like a notebook than a full-fledged arcane tome. Clearly written instructions inside the front page explain the item's purpose to any interested reader. Anyone reading these instructions must decide immediately whether or not to employ the tome's power; if they choose not to do so, it closes itself to that individual forever.

Source
Kael and Lady Vashj can buy these from Goblin Merchants in The Search for Illidan, chapter 4 of the Frozen Throne alliance campaign, Curse of the Blood Elves.