The term pentateuch is derived from the Greek πεντατευχος which refers to a five volume book. The Jews call these five books the Torah, which means “instruction” or “law.” This is why in the New Testament the first five books are often referred to as the “law.”
The names of the books come from the Septuagint (a Greek translation of the Old Testament dating from the third century BC) via the Vulgate (the Latin translation of the Bible made by Jerome in the fourth century AD).
Leviticus—“having to do with the Levites”
Numbers—i.e. of the census
Deuteronomy—“the second law”
Genesis and Deuteronomy are clearly independent books. Genesis tells the story of the creation of the world and God’s dealings with his people up to the time of Joseph and the descent into Egypt. Deuteronomy is essentially Moses’ last speech to the sons of Israel before his death. The end of Deuteronomy records Joshua’s succession to Moses and the circumstances of Moses’ death.