The Memoirs of Jacintus is the detailed recollections of a Wanderer (former Angel) named Jacintus Magistra. He was the Kepper before the prodigal Mithral who started the war. The book was in a library in Azrath which Raven has salvaged before her arrival on Earth. The memoir (when inactive) is less thicker than a magazine. When it is activated, the hidden pages become visible and Jacintus' Presence is awakened where he can assist you through the vast contents of the book.
Though one of the smartest beings in that Universe, Jacintus is exceptionally CARELESS. Even giving to Raven the wrong page of the book when Azriath asked him to show her how to make a Shietxu, he showed her how to make a stronger one, a Shenzei which caused Raven to pass out after the battle.