Tome of Intelligence

Permanently increases Intelligence.

Source
Arthas and Kel'Thuzad find two in Blackrock & Roll, Too!, and one in The Siege of Dalaran, chapters 6 and 7 of the Warcraft III undead campaign, Path of the Damned.

Thrall can find one in Cry of the Warsong, and one in The Oracle, chapters 3 and 7 of the orc campaign, The Invasion of Kalimdor.

Maiev can find one in Rise of the Naga, three in The Broken Isles, and one in The Tomb of Sargeras, chapters 1, 2 and 3 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael can find two 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 one in King Arthas, chapter 1 of the Frozen Throne scourge campaign, Legacy of the Damned.