The book of Joshua begins where the book of Deuteronomy ends, with the death of Moses (see Deuteronomy 34). This is just the first instance of many links between the two books, hence, the scholars' designation of the author of Joshua as the "Deuteronomist."
After the illustrious career of Moses, "the servant of the LORD" (1:1), Joshua is to take over leadership of the Israelites and lead them at last into Canaan, the land God has promised to them. This entry into the land fulfills the promises of the whole Pentateuch--promises to Abraham, Isaac, and Jacob, as well as promises to Moses and the first generation of Israelites in the wilderness. As such, the book of Joshua records a momentous time in Israel's history.
The task of leading the people at such a time would seem daunting to anyone. Perhaps that is why Joshua is told by God three times (1:6, 7, 9) and by the people once (1:18) to "be strong and courageous."
The Lord also commands Joshua to obey the book of the law of Moses (1:7-8). The phrase "book of the law" refers to Deuteronomy, or at least some part of Deuteronomy (see Deuteronomy 31:9-13, 24-26). Joshua reads this book of the law to the people in a covenant renewal ceremony in Joshua 8:30-35 (see also 24:25-27).
1After the death of Moses the servant of the Lord, the Lord spoke to Joshua son of Nun, Moses assistant, saying, 2My servant Moses is dead. Now proceed to cross the Jordan, you and all this people, into the land that I am giving to them, to the Israelites. 3Every place that the sole of your foot will tread upon I have given to you, as I promised to Moses. 4From the wilderness and the Lebanon as far as the great river, the river Euphrates, all the land of the Hittites, to the Great Sea in the west shall be your territory. 5No one shall be able to stand against you all the days of your life. As I was with Moses, so I will be with you; I will not fail you or forsake you. 6Be strong and courageous; for you shall put this people in possession of the land that I swore to their ancestors to give them. 7Only be strong and very courageous, being careful to act in accordance with all the law that my servant Moses commanded you; do not turn from it to the right hand or to the left, so that you may be successful wherever you go. 8This book of the law shall not depart out of your mouth; you shall meditate on it day and night, so that you may be careful to act in accordance with all that is written in it. For then you shall make your way prosperous, and then you shall be successful. 9I hereby command you: Be strong and courageous; do not be frightened or dismayed, for the Lord your God is with you wherever you go.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org