A publication for the first time of three texts written by Felix De Andreis, first superior of the Vincentians (Congregation of the Mission) in the United States. 1) The Indian Grammar is a grammar and dictionary that De Andreis drew up to help him in the evangelization of Native Americans in Missouri. The original had been lost since his death in 1820. 2) "A New Catechism" is an unknown document prepared by De Andreis to aid in the conversion of non-believers. Its identity was recently confirmed. 3) A fragment of an unknown letter by De Andreis from 1819, consisting of a single sentence. -- Each of the texts is transcribed in whole or in part, followed by commentary.