Celie is uneducated, and she is writing exactly as she speaks and thinks. There is nothing artificial about her writing "style." In fact, the most distinctive characteristic about Celie's letters is their naturalness. There is a continuous emphasis on the oral sound and sense of what Celie writes, rather than on the "written" style of the letters.
There is also a keen and enduring quality of honesty throughout Celie's letters. She is writing to God, trusting him as she would trust a best friend for guidance and strength to carry on, despite the terrible, painful unhappiness that she feels within her and all those around her.
You should also note that Celie doesn't sign her letters for a long time, which can be explained by realizing that Celie doesn't think of herself as a person of sufficient worth to sign her name. When we meet Celie, she has very little self-confidence. She feels unloved. No one has made her feel valuable. Thus, she turns to God. But even in God's company, Celie feels of little worth.
It will be a long time before Celie gains enough self-esteem to sign her name with pride, but by then, we will have realized that in reading this long series of letters, we have witnessed a wondrous growth of a black woman who was born with all the odds against her. She began life as a virtual slave, the victim of men, of traditional sexual roles, of racism, and of innumerable social injustices. When the novel is finished, we will have seen Celie grow into a whole human being — as well as into a mature, twentieth-century woman.
