“The ‘book of Jashar’ is mentioned in the Old Testament. Can you explain what this book was?”
The “book of Jashar” (ASV) is referenced twice in the literature of the Old Testament; in Joshua 10:13 and in 2 Samuel 1:18. We will consider each of these episodes and its relationship to this ancient document.
(1) After Joshua, the successor of Moses, had conquered the cities of Jericho and Ai, out of fear the community of Gibeon made an alliance with Israel. When he learned of the association, Adonizedek, king of Jerusalem, gathered together a confederation of rulers who commenced an attack upon Gibeon. The men of Gibeon sent for Joshua, pleading for his help against these hostiles.The Israelite commander came with all dispatch and engaged the enemy at Beth-horon, about 12 miles NW of Jerusalem. What followed was one of the greatest battles of history.
Jehovah joined forces with the Hebrews, raining great hailstones from the sky, and the heathen forces were slain mightily. In order to extend the time of the “mopping up” effort, Joshua petitioned the Lord, saying, “Sun, stand still at Gibeon, and moon, in the Valley of Ajalon.And the sun stood still, and the moon stopped, until the nation took vengeance on their enemies.” Subsequently, the writer (most likely Joshua himself) asked, "Is not this written in the book of Jashar?
(2) The second reference to the book of Jashar is found in David’s lament for Saul and Jonathan following the deaths of these two (1 Samuel 31). The father had made himself an enemy of the shepherd lad, and the son became David’s closest friend. The poignant ode is recorded in 2 Samuel 1:19-27, but the author of 2 Samuel cites the book of Jashar as the earlier source from which the poem was taken.
The Book of Jashar
The expression, “book of Jashar,” translates two Hebrew words that signify “book of the upright” or “book of the righteous one.” Apparently it was an ancient Hebrew hymnbook containing nationalistic songs. The Syriac translation of the text in Joshua calls it the “book of praises,” or “book of hymns.” The references in the books of Joshua and 2 Samuel would suggest that this collection of songs were compositions designed to celebrate great battles and notable characters in Israel’s illustrative history, as the nation prepared for the coming of the Messiah.
A text in 2 Samuel may suggest that the book was used as a training text for Israel’s military forces. The passage reads: “and he bade them teach the children of Judah the song of the bow: behold it is written in the book of Jashar” (1:18 ASV). One scholar, R.A. Mitchell, of the Institute of Mediterranean Studies (Berkeley, California), has paraphrased the text as follows: “He instructed them to train the Judeans in bowmanship, the training-poem for which is written in the book of Jashar” (Wycliffe Bible Dictionary, C.F. Pfeiffer, Howard Vos, John Rea, eds., Peabody, MA: Hendrickson, 2003, p. 884).
The book of Jashar is no longer extant. A copy, circulating under that name since 1751, has been determined to be a forgery.