The Book of Jashar is also mentioned in Joshua 10:13. It is understood to be a Scroll of the Righteous
 that deals with the heroic exploits of the Israelites.1 The fact that this lament is found in this book suggests that it has something to do with preparing Israelites for (future) acts of heroism.
18 and he said it should be taught to the people of Judah; behold, it is written in the Book of Jashar. He said: