Tome of Knowledge (Warcraft III)

Permanently increases Strength, Agility and Intelligence.

Source
Arthas and Kel'Thuzad find one in Blackrock & Roll, Too!, chapter 6 of the Warcraft III undead campaign, Path of the Damned.

Tyrande can find one in Brothers in Blood, chapter 5 of the night elf campaign, Eternity's End.

Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of the Frozen Throne alliance campaign, Curse of the Blood Elves.