A Summoner's Journal is found in Act 1.
Location:
Southern Highlands Near the East there will be a single cultist chanting on lower ground with four corpses surrounding him.
Quest:
Any Quest
"Maghda is a fool. I will raise an army of souls from these primitives and chain them to my invincible will. Yes, with these heathens at my command, the kingdoms of the world shall bow to the might of... The Lord of Goats! I may have to work on the title a bit more. - Dark Cultist"