The Old Testament refers to the “book of Jasher” twice. In Joshua 10:13↗, after recounting the day the sun “stood still” at Gibeon, the narrator asks, “Is not this written in the book of Jasher?” In 2 Samuel 1:18↗, David’s lament for Saul and Jonathan (“the bow”) is said to be taught from the same source. Both references treat the book as a known and trusted reference — but the work itself is not preserved. It is one of several lost books named in the Old Testament, alongside the Book of the Wars of the LORD (Numbers 21:14↗) and the chronicles of the kings of Israel and Judah.
A separate Hebrew work called Sefer haYashar (“The Book of the Upright”) circulated in the Jewish world from the medieval period onward. It is a sweeping retelling of biblical history from creation through the time of the judges, drawing on rabbinic legend and midrash. It bears the same Hebrew title as the lost original, and several English translations since the 19th century have made it accessible. But the medieval Sefer haYashar is not the work Joshua and 2 Samuel cite; it is a later composition that took on the same name.
For the Latter-day Saint reader, the lost books of the Old Testament are not a marginal curiosity. They figure in Latter-day Saint thinking about the canon’s openness and the principle of continuing revelation — that Scripture, as it has reached us, is not the whole of what God has caused to be written. The Jasher references are among the clearest in-canon acknowledgments of that.