The Book of Deuteronomy (Greek for "second law") is the last book of the Torah. Moses has guided the Children of Israel to Moab, to the East Bank of the Jordan River, and they are ready to cross the river and enter the Promised Land. God has told Moses that he must ascend the mountain on that bank, and die, and that Joshua, son of Nun, will lead the Israelites across the river to their destiny. As a final gesture, he assembles the Children of Israel and makes one last great speech, which is the Book of Deuteronomy.
The Documentary Hypothesis suggests that Deuteronomy was written by a different author than the rest of the Bible, a post-exilic author. It certainly has a different feel than the other books of the Torah. About 70% of the book is a repeat of things in the rest of the Torah, with about 30% new information. The Ten Commandments are repeated here, but are subtly different. Often the rules repeated here have a slightly different spin on them.
The book starts with Moses describing their journey of the last forty years. He does not paint a flattering picture of their conduct, showing how their bickering, idolatry and faithlessness is what contributed to their misfortune. He tells the story about the spies who were chosen to enter the land and report back to the Israelites, and how their faithlessness condemned their generation to die in the wilderness [Deuteronomy 1: 35-39], with the exceptions of Joshua and Caleb. The date of this condemnation is taken to be Tisha B'Av, which is coming up this Saturday (but because it falls on Shabbat this year, will be observed on Sunday).
There is a midrash that at each new camp in the Wilderness, God commanded each of the generation that left Egypt to dig their own graves and lie in them. Each night, they would line up in front of their individual graves, enter them and lie down inside the grave. And each time this was done, a few of them did not emerge from their graves. After 40 years, the graves had consumed them all. By the time they reached the end of the Book of Numbers, none of that generation were left.
Because Moses is addressing a generation of people born in the Wilderness who never knew Egypt, he has to restate the rules to this new generation. He knows that when he finishes talking, he has to die, so Deuteronomy has an urgency as Moses tries to tell these people everything he possibly can that he feels that they need to know.