This book is called Deuteronomy, that is, the second Law. For thus Moses himself calls it in Deut. 17:18 ff., where he commands him who becomes King of Israel to read in this second Law every day when he has received it from the Levites; and in Deut. 31:10 ff. he commands that every seven years, when the release is celebrated, it be read before all Israel at the Feast of Tabernacles at the place which the Lord has chosen. It is called “second,” however, not because it is different from that which was given on Mt. Sinai but because it was repeated by a new covenant through the ministry of Moses in the presence of those who had not heard it the first time on Mt. Sinai. For all who had heard it from the Lord Himself had perished in the desert.
Hence this book is a summary of Moses and a brief of the total Law and wisdom of the people of Israel. In it all those things which pertain to the Levites and priests are omitted, and only those things are taught which need to be known by the people and the common folk. If you want to give this book a name suitable for our use, you will correctly call it a most ample and excellent explanation of the Decalog. After you know it, you could want nothing more that is needful for understanding the Ten Commandments. For it teaches this people to live well according to the Ten Commandments in both spirit and body. Furthermore, it sets up the inner rule of conscience together with secular government, then the outward manner of ceremonies with wholly divine justice and wisdom, so that there is nothing in the whole range of life that is not arranged here most wisely and properly.
Luther’s works, vol. 9: Lectures on Deuteronomy 1525