Tome of Strength +2

Permanently increases the Strength of the Hero by 2 when used.

Source
Arthas finds one in The Culling, chapter 6 of the Warcraft III human campaign, The Scourge of Lordaeron.

Arthas can also find one in Under the Burning Sky, chapter 8 of the undead campaign, Path of the Damned.

Malfurion, Tyrande and Maiev can find one in Balancing the Scales, chapter 5 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.

Illidan, Kael, Lady Vashj and Akama can find two in Lord of Outland, chapter 6 of the alliance campaign, Curse of the Blood Elves.