Archangel Malthael, the Aspect of Wisdom is a tome found in Act IV of Diablo III.
It reads as follows:
Much uncertainty surrounds the archangel Malthael. Once the calm guiding hand of wisdom, he was lost to us the moment the Worldstone disappeared. Unable to fathom that catastrophic event, he abandoned Heaven. His departure created a colossal fracture within the council, one that, to this day, remains unrepaired. - Selathiel, Angiris Scribe