31:19 write ye this song. Moses had frequently both reminded the people and warned the people, but he knew (and God told him) that they would forget. They would listen more attentively and remember longer if it was part of a song that could be sung generation after generation.
31:24 in a book. This is a clear assertion that Moses, not some later combination of “redactors,” was the direct author of all the words of the law, including the complete books of Exodus, Leviticus, Numbers and Deuteronomy. Because Genesis was compiled and edited by Moses from the writings of the earlier patriarchs, it is also normally included with the others as one of the books of the law, all now called the Pentateuch. The entire “book” was then placed in the ark of the covenant to be a perpetual witness to the people (Deuteronomy 31:26).