A tome said to contain the true name of every demon in existence. Varedis power may be connected to the book, as ripping the pages from it takes some of his power away.
This quest item drops from Blackheart the Inciter when players are on the quest  .
Book of Fel Names as a quest objective
The Book of Fel Names is nearly inspired in H.P. Lovecraft's Necronomicon, which means Book of the Dead Names.