What is “the book of Jashar” mentioned in Joshua 10:13?

“Jashar” is a Hebrew word which literally means “upright man” or “just man.”  The book of Jashar is mentioned not only in Joshua in the context of being another source of record of the episode of the sun standing still, but also in 2 Samuel 1:18 in the context of being another source of record of David’s song of mourning over the deaths of Saul and Jonathan.

Inspired Scripture has been preserved by the providence of God so that we who lived thousands of years later could have it.  The same cannot be said of the book of Jashar.  All we know is that evidently it once existed and seemed to be one of the sources used by the authors of Joshua and Samuel.

Since it is said to record David’s song of poetry about the deaths of Saul and Jonathan, some believe it to be a book of poetry.  It is also thought that the reason Joshua mentioned it while talking about the sun miraculously standing still was to show readers who might be skeptical as to the veracity of that even that other people besides himself accepted and recorded it.  By basically pointing to the book of Jashar and saying, “You can read about it here too!”, he was authenticating the event.