St. Louis Marie wrote several private letters during his lifetime, 34 of which (or some of them) have been preserved. Letters were written to 14 recipients, the main ones being Fr. M. Leschassier (his confessor and spiritual director), his sister Guyonne Jeanne, and Sr. Marie Louise of Jesus (Marie Louise Trichel). The letters help us to understand him as a writer and what he wrote in his works. Some of them (especially Chapters 5. 6, 8-11, and 15-16) highlight the early period of his priestly life, when he was struggling to find God's Will for him.