The Book of Jasher also known as Book of the Just Man or the Upright One is an unknown book referenced in the Old Testament of the Bible. While the book is mentioned in the Bible it is not a canonical member of the books of the Bible.
The book is referenced as a poetical or historical record in both the Book of Joshua and in Second Samuel. It is likely that book was either a poetic or historical record of the actions of Israelite war heroes during the conquest of Canaan and the battles they participated in. The book being quoted twice was likely a familiar historical work amongst Hebrews. Being recited by two biblical books, and likely being widely recognized among the Israelite culture the book was probably written by Israelites and would retain historical accuracy and compliance with biblical documentation.
The name of "Jasher" (or Jashar) is fully translated to be "the Just Man" or "the Upright One, which seems to be a reference to the warriors in the book being "just". The name is singular, however, when the book probably contains information on multiple warriors.
The book must have been continuously updated, because orations given in two separate time periods are both recorded. First in the Book of Joshua, Joshua recited a victory chant to God after the Amorites were defeated. While Joshua is the author, it mentions the his victory chant as "being written in the Book of Jashar", implying that the book existed before the Book of Joshua (which may have been written much later after the events).
Second, after the death of Saul and Jonathan David recited a lament of the bow (played on the bow). He then ordered that all the people of Judah, were to be taught the lament. Here in Second Samuel it mentions that the lament is also written in the Book of Jasher. This gives evidence that it was updated to include notable warriors of the different time periods of Israel. At the time of David's lament, it would have been someone living in Judah who added the lament to the book.
The criteria for inclusion in the book is unknown, but it is known that Joshua, Saul and Jonathan all have poetry or narratives recorded about them.