Manual of Health

Permanently increases the hit points of the Hero by 50 when used.

Source
Thrall finds one in Chasing Visions, chapter 1 of the Warcraft III prologue campaign, Exodus of the Horde, by defeating a forest troll.

Arthas and Muradin Bronzebeard find one in The Shores of Northrend, chapter 7 of the 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 also find one in Landfall, chapter 1 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Daughters of the Moon, chapter 2 of the night elf campaign, Eternity's End.

Tyrande and Furion can also find one in Brothers in Blood, and one in Twilight of the Gods, chapters 5 and 7 of that campaign.

Illidan can find one in A Destiny of Flame and Sorrow, chapter 6 of that campaign.

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

Kael can find one in Misconceptions, chapter 1 of the alliance campaign, Curse of the Blood Elves.

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