Archangel Imperius, the Aspect of Valor is a tome found in Act IV of Diablo III, specifically within a Hell Rift.
It reads as follows:
Imperius, the archangel of Valor, is the greatest warrior in all of creation. He revels in war and combat and has led the hosts of Heaven to innumerable victories. With Malthael's absence, it is Imperius who now rules over the Angiris Council, seeking always to defend the High Heavens from any foe. - Selathiel, Angiris Scribe