Adding new effects
Effects are things like sickness, poison, or drug effects. They’re also generally useful for applying a status condition on a monster or NPC that you go back and check later.
Effects are mostly stored in data/json/effects.json, and described in https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/EFFECTS_JSON.md
Books and comestible (food/drink/medicine) items can have morale modifiers. I’m not sure if there’s a general system for morale modifiers or not.