The Book of Jasher was a historical record referenced in Joshua and 2 Samuel but is now lost. Though later writings claimed its name, none are the original or inspired by God, and the Book of Jasher was never part of the biblical canon.
The Book of Jasher, mentioned twice in the Old Testament (Joshua 10:13 and 2 Samuel 1:18-27), was an ancient historical record that documented notable events, such as Joshua’s military victory and David’s lament for Saul and Jonathan. While its original text is now lost, later works claiming the name “Book of Jasher” are not connected to the biblical references and are not divinely inspired. The Bible references it as a supplementary source, but it was never regarded as Scripture, nor is there any indication that it was meant to be included in the biblical canon. Its mentions serve to confirm historical events rather than provide authoritative teaching, and even if it existed today, it would not replace or add to the inspired Word of God. The Book of Jasher remains historically interesting but is ultimately secondary to the canonical Scriptures.
In the 1700s a Book of Jasher was published that was alleged to be a translation of the Book of Jasher by Alcuin, an eighth-century British writer. Another work named Pseudo-Jasher and written in Hebrew is dated to the 1600s but is also not connected with the version mentioned in the Old Testament. Though its claim to be the original is inaccurate, its introduction notes other attempts at a Book of Jasher "by Zerahiah Ha-Yevani of the 13th century. There is also known to have been one written by Rabbi Jacob ben Mier of the 12th century, and one by Rabbi Jonahben Abraham of Gerona of the 14th century. We are told of a work by that title from the Amoraim period (3rd to 6th centuries) that is characterized as containing 'for the most part sayings of the sages of the first and second centuries.'" Again, however, the original work of the Book of Jasher is apparently lost and is now only known through its two references in the Bible.
Because the Book of Jasher no longer exists, it clearly cannot be part of the Bible. Yet even if it did exist today, there is no reason to believe it would need to be added to the Bible. The Old Testament writings were already affirmed and had been translated into Greek before the time of Christ. No biblical writer claimed the Book of Jasher was divine; only that it served as another source to confirm two biblical references.