Tome of Intelligence +2

Permanently increases Intelligence.

Source
Maiev can find one in Rise of the Naga, and one in The Tomb of Sargeras, chapter 1 and 3 of the Frozen Throne sentinels campaign, Terror of the Tides.

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

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