How the bestiary is organised in The Witcher 3
Every creature with a bestiary entry in The Witcher 3, grouped by the class the game files put it in, which is the same class its oil is made for. Each page names the oil to coat a blade with, the decoction and trophy it yields, what it drops, and the stats the game defines for it.
Reading the class first is the shortcut: one oil covers every monster in it, so knowing a katakan is a vampire matters more than knowing its health. Oils and decoctions are listed under Witcher 3 alchemy.
Reading a Witcher 3 monster entry
Each Witcher 3 bestiary entry carries the hunting clues the game shows before a fight, the abilities the monster uses, and what it is weak to beyond the oil, which is usually a sign or a bomb rather than a sword. Necrophages, specters and relicts all fight differently enough that the class alone tells you how to open.
Drops are listed because most crafting chains start with a monster part. Anything you cut off a kill turns up again on the crafting components page with the recipes that consume it.