The Book of Numbers (or simply, Numbers) is the fourth book of the Pentateuch in the Old Testament of the Bible. Numbers continues the narrative of Exodus and Leviticus. In particular Numbers documents the Nation of Israel officially allocating areas to the twelve tribes through both historical accounts and records. Numbers also documents several rebellions against the leadership of Moses and God over Israel. Numbers also documents Israel's wandering in the desert for forty years, as a result of refusing to enter Canaan; due to presence of the Nephilim.
The Book of Numbers was written by Moses.
In large part, Numbers is composed of various types of records. Most of the records are concerned with the tribes of Israel and specifics about their numbers and involvement in various events. One of the most notable example of this is the census of the tribes of Israel recorded in the book. The records are usually accompanied by relevant historical context or laws.
The records included in Numbers are: sacrifices given by the tribes for the Tabernacle, the positions of the tribes around the Tabernacle, scouting of Canaan, correspondence with Edom and Moab, Balaam's prophecies, battle spoils, other censuses, locations of rest after the Exodus and land divisions.
It is highly unlikely that any of these records were original to Numbers. Rather they would have been added to the Book of Numbers under Moses' direction. These records would have likely been recorded (unless memorized orally) beforehand.