These letters are structured with irony. It is painfully ironic that Sofia leaves prison only to become the one thing that she absolutely refused to become: a white woman's maid. And the irony is compounded by the fact that she must watch over Miss Millie's children and not her own. Prison may have been hard on Sofia, but, ironically, being a "maid" is far harder on her, psychologically. Isolation in prison gave Sofia enough time to reflect on her situation; she has been a victim of both racism and sexism. Her life at present is a kind of non-life. She has looked at the abyss of her future, and like one of Camus' characters, she has seen the absurdity of living, even though she does continue to live. She knows that she and Miss Millie can never have the kind of relationship that two women should have; they can have a relationship based only on differences and protocol — the antitheses of Sofia's sisterhood with Celie, Squeak, and Odessa.
The white community refers to Sofia, a woman with six children, as a "girl." Yet, Sofia realizes ironically how "girl-like" and ignorant Miss Millie is, while, at the same time, Sofia has to act as if Miss Millie were superior. Camus' world of the absurd is lived in, day-to-day, as if rules, customs, and observances mattered — while knowing, at the same time, how hollow they are. Sofia knows how thoroughly immersed in racism Miss Millie is, and she also knows how terribly deluded the "do-good" white woman is. It is little wonder that Sofia is relieved and happy to admit that she is glad that she is "not white."
As a small footnote, you should be aware that ever since Letter 36, we have seen the development of cars and their being integrated into the daily lives of rural blacks. Sofia's prizefighter has a car, and, of course, Miss Millie has one. Shug has a fancy, queenly dark blue Packard for herself and Grady. These cars are a symbol of mobility as well as a symbol of progress. The wagons of the early letters seem to be gone. The years are rolling by, and Celie, whom we met at fourteen, is now approaching middle-age.