Jean (Jehan) Héroard was personal physician to Louis XIII of France. Born in 1551, Héroard was fifty years old at the time of Louis' birth and kept a daily journal of Louis's development from birth for 26 years, terminating only due to Héroard's own death in 1628.

This journal, along with commentaries, is reproduced in the following publication:
Foisil, Madeleine (Ed.) 1989. Journal de Jean Héroards. Publication du centre de recherche sur la civilisation de l'Europe moderne, 2 volumes, Paris, Fayard (Vol 1, 1601-1608; vol 2, 1609-1628)

Additional references include:

Ernst, G. (1985) Gesprochenes Franzoesisch zu Beginn des 17
Jahrhunderts, Direkte Rede in Jean Héroards "Histoire particulière de Louis XIII" (1601-1610), Tubingen, Max Niemeyer Verlag, X-623pp.

Gougenheim, H (1931) L'observation du langage d'un enfant royal au XVIIe siècle. Revue de philologie française et romane, 43, 1-15.

Ingram, D. & Le Normand, M-T. (1996). A diary study on the acquisition of Middle French: A preliminary report of the early language acquisition of Louis XIII. Proceedings of the 20th annual Boston University conference on language development, 352-63.