What is a person called when he researches someones life and records it in book?

There are a few terms for someone who researches and writes about someone else's life, depending on the context and type of writing:

* Biographer: This is the most common and general term for someone who writes a biography, a detailed account of a person's life.

* Author: This is a broader term, as it can apply to any writer, but it can also be used specifically for someone who writes a biography.

* Historian: This term applies when the subject's life is being studied within a historical context.

* Chronicler: This term is more specific to someone who records events in a chronological order, often for a specific time period or person.

* Memoirist: This term applies if the person writing is a close acquaintance or family member of the subject, and the work is written from a personal perspective.

* Scholar: This term can be used when the biographer is conducting academic research and writing.

The best term to use depends on the specific nature of the research and writing.