The Question and Answer section for The Color Purple is a great resource to ask questions, find answers, and discuss the novel.
Celie makes Nettie promise to write.
She does not seem too emotionally or intellectually intelligent. She marries the abusive Pa and does not believe his abuse even while being abused.
From the text:
"Miss Beasley always said I was the smartest child she ever taught! But one thing I do thank her for, for teaching me to learn for myself, by reading and studying and writing a clear hand."