From the Times Literary Supplement:
Flaubert is exemplary, indeed talismanic, for the stern separation he made between his public and private writings. His novels are objective constructions which unfold in authorial absence; his letters are a place of riotous opinion-giving and frank emotional unbuttoning. Yet the distance between the two was not empty but connective. It was part of Flaubert’s literary strategy to treat his correspondence as a déversoir, an overflow, an outlet which purged the intrusive self and helped liberate the fiction into its desired impersonality. Three years before Madame Bovary appeared, he bade farewell, in a letter to Louise Colet, to “the personal, the intimate, to everything connected with me”. His “old project” of one day writing his memoirs was now officially abandoned: “Nothing personal tempts me any more”.
This makes such overflow as we have the more fascinating: the incomparable letters, but also the travel notes and the Cahiers intimes of 1840–41.