Deuteronomy 31:9

The law was given by Moses; so it is said, Joh 1:17. He was not only entrusted to deliver it to that generation, but to transmit it to the generations to come; and here it appears that he was faithful to that trust. I. Moses wrote this law, Deu 31:9. The learned bishop Patrick understands this of all the five books of Moses, which are often called the law; he supposes that though Moses had written most of the Pentateuch before, yet he did not finish it till now; now he put his last hand to that sacred volume. Many think that the law here (especially since it is called this law, this grand abridgment of the law) is to be understood of this book of Deuteronomy; all those discourses to the people which have taken up this whole book, he, being in them divinely inspired, wrote them as the word of God. He wrote this law, 1. That those who had heard it might often review it themselves, and call it to mind. 2. That it might be the more safely handed down to posterity. Note, The church has received abundance of advantage from the writing, as well as from the preaching, of divine things; faith comes not only by hearing, but by reading.

— Matthew Henry's Concise Commentary (public domain)

HE DELIVERS THE LAW TO THE PRIESTS, TO READ IT EVERY SEVENTH YEAR TO THE PEOPLE. (Deu 31:9-13) And Moses wrote this law, and delivered it unto the priests--The law thus committed to writing was either the whole book of Deuteronomy, or the important part of it contained between the twenty-seventh and thirtieth chapters. It was usual in cases of public or private contract for two copies of the engagement to be made--one to be deposited in the national archives or some secure place for reference, should occasion require. The other was to remain in the hands of the contracting parties (Jer 32:12-14). The same course was followed on this renewal of the covenant between God and Israel. Two written copies of the law were prepared, the one of which was delivered to the public representatives of Israel; namely, the priests and the elders. the priests, . . . who bare the ark of the covenant--In all ordinary journeys, it was the common duty of the Levites to carry the ark and its furniture (Num 4:15); but, on solemn or extraordinary occasions, that office was discharged by the priests (Jos 3:3-8; Jos 6:6; Ch1 15:11-12). all the elders of Israel--They were assistants to the priests and overseers to take care of the preservation, rehearsal, and observance of the law.

— Jamieson-Fausset-Brown Commentary (public domain)