The Book of Jasher is mentioned twice in Scripture (Joshua 10:13 and 2 Samuel 1:18) and was considered by both Joshua and Samuel to be a reliable source of information. The web pages of this section record what is commonly believed to be that very same Book of Jasher.
Do we know this with certainty? No, not with the information currently available to us.
Please note that, while the Book of Jasher is mentioned in Scripture, the book itself is not Scripture. It is, however, a valuable work of historical and apocryphal literature.
THE BOOK OF JASHER
THIS IS THE BOOK OF THE GENERATIONS OF MAN WHOM GOD CREATED
UPON THE EARTH ON THE DAY WHEN THE LORD GOD
MADE HEAVEN AND EARTH.
CHAPTER 1--The Creation of Adam and Eve. The Fall. Birth of Cain and Abel. Abel a Keeper of Sheep. Cain a Tiller of the Soil. The Quarrel Between the Brothers and the Result. Cain, the First Murderer, Cursed of God.
CHAPTER 2--Seth is Born. People begin to Multiply and Become Idolatrous. Third Part of the Earth Destroyed. Earth cursed and becomes corrupt through the Wickedness of Men. Cainan, a Wise and Righteous King, Foretells the Flood. Enoch is Born.
CHAPTER 3--Enoch Reigns over the Earth. Enoch Establishes Righteousness upon the Earth, and after Reigning Two Hundred and Forty Years is Translated.
CHAPTER 4--The People of the Earth Again Become Corrupt. Noah is Born.
CHAPTER 5--Noah and Methuselah Preach Repentance for One Hundred and Twenty Years. Noah Builds the Ark. Death of Methuselah.