The Book of Jashur (Jasher) or literal translation being “The Book of the Upright”, is an incredible extra-biblical work of unknown origins and authorship, possibly passed down from the righteous to the righteous as history occurred.
There are arguments about how it came into the present day with one side being that it was smuggled away prior to the destruction of the 2nd temple by the Romans, and kept for some time before being published. And the other being that it was wholly made up after the fact.
Regardless it is an incredible work, either as story or history. I’ll leave it to you to decide:
In my opinion it is valid as an historical work, possibly even the basis for Moses’ writings regarding The Creation Story, however I would always subject it to the preeminence of the Biblical Cannon of scripture. Enjoy…
Here is another version: