|Easton's Bible Dictionary|
Upright. "The Book of Jasher," rendered in the LXX. "the Book of the Upright One," by the Vulgate "the Book of Just Ones," was probably a kind of national sacred song-book, a collection of songs in praise of the heroes of Israel, a "book of golden deeds," a national anthology. We have only two specimens from the book, (1) the words of Joshua which he spake to the Lord at the crisis of the battle of Beth-horon (Joshua 10:12, 13); and (2) "the Song of the Bow," that beautiful and touching mournful elegy which David composed on the occasion of the death of Saul and Jonathan (2 Samuel 1:18-27).
Int. Standard Bible Encyclopedia
JASHER, BOOK OF
ja'-sher, jash'-er: the King James Version for JASHAR (which see), and see BETH-HORON, THE BATTLE OF.
Jasher (2 Occurrences)
Joshua 10:13 And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day. (KJV DBY WBS)
2 Samuel 1:18 (Also he bade them teach the children of Judah the use of the bow: behold, it is written in the book of Jasher.) (KJV DBY WBS)