Tome of Agility +2

Permanently increases Agility.

Source
Malfurion, Tyrande, and Maiev can find two in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.

Kael and Lady Vashj can find one in A Dark Covenant, chapter 2 of the alliance campaign, Curse of the Blood Elves.