The Book of the Law of Moses refers to the details and terms of the covenant God made with Moses at Mount Sinai. Most likely it included material from Exodus, Leviticus, and Deuteronomy—though Deuteronomy was probably the focus. In mentioning this book, Joshua is expanding the command given to him by the Lord (Joshua 1:6–10) to the rest of the Israelites.
6 Therefore, be very strong to keep and to do all that is written in the Book of the Law of Moses, turning aside from it neither to the right hand nor to the left,