File Off the Serial Numbers is a d20 sourcebook giving advice and examples for using existing monster stat blocks in new "roles" as NPCs. For example:
Use a young black dragon's stat block as a CR 7 lizardfolk sorcerer
Use a dark creeper's stat block as a CR 2 rogue
Use a succubus's stat block as a CR 7 enchanter
Use a xill's stat block as a CR 6 monk or rogue
Use a yeti's stat block as a CR 3 barbarian
This PDF gives over 80 examples of how to use existing monster stat blocks as humanoid NPCs, explains how to quickly swap or ignore irrelevant monster abilities, and includes tables quickly summarizing roles by CR, class, and creature size. The point of File Off the Serial Numbers is to get more use out of the monster stat blocks you already have, and save you time when you quickly need an NPC or adversary when the PCs take an unexpected turn.