What almost every writer has researched

- wilderness survival skills

- Cultures around the world

- sicknesses

- how much of something the human body can take before dying

- synonyms for literally any word

- descriptive words

- the phycological impact of situations

- names

- words to use instead of ‘said’

- what was popular each decade or era

- clothing trends

- things about animals

- legends and myths

- ways to kill things and how to dispose of bodies

- rhyming words

Comment more that I missed. Am I wrong?