This quest starts by returning to Arbiter Selek. You must wait 2 hours and 24 minutes (2 in game days = 2x72 minutes) after completing the previous quest before he will give you this quest.
Arbiter Selek will send you to kill Sothis and examine a book in Halls of Fate.
The book is in the same room as Sothis. As you enter the room if you head down the right walkway it is leaning against the walkway at ( -2, -69, 310 ) /waypoint -1.88, -68.96, 310.