The book dropping off Lady Victoria
The Horrible Secret is a tome found in Act V of Diablo III and is required for the People Finder lorebook achievement.
The tome is found in the House of Deep Sorrow (found in Westmarch Commons). It drops off the body of Lady Victoria after she is killed.
It reads as follows:
The angels and their monsters are killing everyone! What will they do to me when they find out what I truly am? My mother and the demon attack she survived. It's ... it's all too horrible to contemplate.