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 (WC3 NightElf), 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.