Consider other novels by Toni Morrison, such as Beloved, Song of Solomon, and Tar Baby. Especially consider The Bluest Eye, which explores similar themes of female sexual freedom and colorism.
Other novels by prominent female African American writers like Alice Walker’s The Color Purple (1982) or Maya Angelou’s I Know Why the Caged Bird Sings (1969) could invite interesting comparisons and provide context for Sula in the broader genre of African American Literature.
Consider other novels on the theme of female friendship in a complex social context, such as Elena Ferrante's Neopolitan Novels.