The "Book of Jasher" you can buy today is the actual lost book quoted in Joshua 10:13 and 2 Samuel 1:18: a real, ancient sacred text that survived outside the Bible

Joshua 10:13 and 2 Samuel 1:18 both reference a source text called the "Book of Jashar" (or Jasher, sefer hayashar, "the Book of the Upright"), quoting brief poetic lines from it: the sun-and-moon poem in Joshua, and David's lament over Saul and Jonathan in 2 Samuel. That this source existed and that Israel's biblical authors could cite outside material without it undermining the Bible's own authority isn't in dispute; several other lost sources are referenced the same way elsewhere in the Old Testament (such as the "Book of the Wars of the Lord" in Numbers 21:14). The book itself, however, has not survived. Nothing that circulates today under the title "Book of Jasher" is that lost text, and the two printed editions people actually encounter have entirely separate, documented, and much later origins.

The first is a genuine literary forgery. In 1751, an English printer named Jacob Ilive published a work claiming to be a translation of the Book of Jasher, made by the 8th-century scholar Alcuin from a manuscript supposedly hidden in a Jerusalem library. It was recognized as fraudulent almost immediately (the contemporary periodical The Monthly Review called it "a palpable piece of contrivance intended to impose on the credulous, and the ignorant"), and Ilive was later imprisoned (on related fraud and heresy charges connected to his publishing activities). This version is sometimes labeled "Pseudo-Jasher" by scholars to distinguish it from the second, more commonly circulated edition today.

That second, more widely reprinted version is the one published in New York in 1840 by Mordecai Manuel Noah, presented as "faithfully translated from the original Hebrew." This English translation corresponds to a real Hebrew work called Sefer haYashar, but that Hebrew text is a medieval Jewish midrashic composition (a retelling of biblical narrative woven with rabbinic legend, covering material from Adam through the early Judges period), not the poetic source quoted in Joshua and Samuel. Scholars who have studied Sefer haYashar's language and content date its composition anywhere from roughly the 9th to the 16th century, with a printed Hebrew edition appearing in Venice in 1625 and internal evidence (including apparent awareness of later rabbinic and even post-biblical historical material) that rules out a Joshua-era or even a genuinely ancient origin. Both the Jewish Encyclopedia and modern scholarly reference works classify it as later midrashic aggadah, valuable as a specimen of medieval Jewish storytelling and biblical elaboration, not as the recovered voice of an Iron Age Israelite source document.