A diarist.
A person who studies or writes history is called a historian.
Jim Benton writes the Dear Dumb Diary series
A person who writes with their left hand is called left-handed or sinistral.
A person who writes laws is called a legislator or a lawmaker.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
Testator.
An author.
a publisist a bigraphy
a playwright
This is a poet
composer