Tome of Agility

Permanently increases Agility.

Source
Arthas and Muradin Bronzebeard find one in Frostmourne, chapter 9 of the Warcraft III human campaign, The Scourge of Lordaeron.

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

Thrall can find one in The Long March, and one in The Oracle, chapters 2 and 7 of the orc campaign, The Invasion of Kalimdor.

Maiev can find one in Rise of the Naga, one in The Tomb of Sargeras, and one in Wrath of the Betrayer chapters 1, 3 and 4 of the Frozen Throne sentinels campaign, Terror of the Tides.

Malfurion and Maiev can also find one in The Ruins of Dalaran, chapter 7 of that campaign.

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 the that campaign.

Kael and Lady Vashj can also find one in The Crossing, the secret level of that campaign.

Illidan, Kael, Lady Vashj and Akama can find four in Lord of Outland, chapter 6 of that campaign.