Barney's Prank Journal is Barney Snodgrass' prison journal in the episode "Fools for Love".
Francis Grimes tells Mark Lilly that the former social service coordinator Barney Snodgrass has escaped prison and left his journal behind which indicates that Barney intends to take revenge on the people in the Department of Integration.
Mark later reads the journal which seems to be written in blood. It reveals that Barney planned to kill those people by pranks. It also details the time when he and Callie Maggotbone used to be lovers before he was caught by Grimes and put away for murdering Erik.
Barney's escape turns out to be a prank Grimes played on Mark. And the journal was actually written by Grimes.