The Book of the Wars of the LORD LORD or Book of the Wars of Yahweh, was a written book that existed during the leadership of Moses over Israel. It is referenced in the Book of Numbers and is used both to explain the Scriptural text and vice versa. While the book is referenced in the Bible, it is not a book of the Bible and there is no evidence that it had the same inspiration from God as the rest of Scripture.
The Book of the Wars of the LORD is most likely a set of poems or written accounts of military conflicts of Israel. Based on the title, everything that is described in the book would be related to military conflicts that were directed and/or fought by God. The title attributes the wars to God, saying that he fought them. This would be in recognition of God's hand in winning a battle or war as he specifically told the Israelites many times in the Pentateuch. It may also be referring to the "armies of Angels" that participated in these battles under the leadership of God. The involvement of Angels in battle may be a spiritual, symbolic, literal understanding; or all three. In attributing the wars to God, the book may also be recording fights that God directed the Israelites to partake in.
Since the book is not contained in Scripture, it may have been a reputed literary work of the time.
The book is referenced once in the Book of Numbers. In the reference the book is quoted directly. In context, the quote follows a record of Israel's settlement activities in Arnon; bordering between Moab and Amor. Numbers references and quotes the book, to explain why the Wars Book details the border of Arnon, Moab and Amor. This would indicate that the book was known well enough to warrant an explanation by Scripture. There may have been some ambiguity regarding the excerpt. The reason for its mentioning in Numbers is unknown and only through speculation can a reason be "known".