Tome of Strength

The Tome of Strength permanently increases the strength of a hero.

Source

 * Arthas and Kel'Thuzad find one in Blackrock & Roll, Too!, chapter 6 of the Warcraft III undead campaign, Path of the Damned.
 * Cairne Bloodhoof can find one in The Oracle, chapter 7 of the orc campaign, The Invasion of Kalimdor.
 * Tyrande can find one in Daughters of the Moon, chapter 2 of the night elf campaign, Eternity's End.
 * Tyrande and Furion can also find one in The Druids Arise, chapter 4 of that campaign.
 * Maiev can find one in Rise of the Naga, and one in Wrath of the Betrayer, chapters 1 and 4 of the Frozen Throne sentinels campaign, Terror of the Tides.
 * Kael can find one in Misconceptions, chapter 1 of the alliance campaign, Curse of the Blood Elves.
 * Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of that campaign.
 * Illidan, Kael, Lady Vashj and Akama can find two in Lord of Outland, chapter 6 of that campaign.
 * Arthas, Kel'Thuzad and Sylvanas can find two in King Arthas, chapter 1 of the scourge campaign, Legacy of the Damned.