Question: "What is the book of Jasher and should it be in the Bible?"
Answer: Also known as the “Book of the Upright One” in the Greek Septuagint and the “Book of the Just Ones” in the Latin Vulgate, the Book of Jasher was probably a collection or compilation of ancient Hebrew songs and poems praising the heroes of Israel and their exploits in battle. The Book of Jasher is mentioned in Joshua 10:12-13 when the Lord stopped the sun in the middle of the day during the battle of Beth Horon. It is also mentioned in 2 Samuel 1:18-27 as containing the Song or Lament of the Bow, that mournful funeral song which David composed at the time of the death of Saul and Jonathan.
The question is, if the Book of Jasher is mentioned in the Bible, why was it left out of the canon of Scripture? We know that God directed the authors of the Scriptures to use passages from many and various extra-biblical sources in composing His Word. The passage recorded in Joshua 10:13 is a good example. In recording this battle, Joshua included passages from the Book of Jasher not because it was his only source of what occurred; rather, he was stating, in effect, “If you don’t believe what I’m saying, then go read it in the Book of Jasher. Even that book has a record of this event.”
There are other Hebrew works that are mentioned in the Bible that God directed the authors to use. Some of these include the Book of the Wars of the Lord (Numbers 21:14), the Book of Samuel the Seer, the Book of Nathan the Prophet, and the Book of Gad the Seer (1 Chronicles 29:29). Also, there are the Acts of Rehoboam and the Chronicles of the Kings of Judah (1 Kings 14:29). We also know that Solomon composed more than a thousand songs (1 Kings 4:32), yet only two are preserved in the book of Psalms (72 and 127). Writing under the inspiration of the Holy Spirit in the New Testament, Paul included a quotation from the Cretan poet Epimenides (Titus 1:12) and quoted from the poets Epimenides and Aratus in his speech at Athens (Acts 17:28).
The point is that the divine Author of the Bible used materials chosen from many different sources, fitting them into His grand design for the Scriptures. We must understand that history as recorded in the Bible did not occur in isolation. The people mentioned in the Bible interacted with other people. For example, though the Bible is clear that there is only one God, the Bible mentions a number of the gods people worshipped both within Israel and in the nations around. Similarly, as in Acts 17:28 and Titus 1:12, we sometimes find secular writers being quoted. This doesn't mean that these quoted writers were inspired. It simply means they happened to say something that was useful in making a point.
There is a book called “The Book of Jasher” today, although it is not the same book as mentioned in the Old Testament. It is an eighteenth-century forgery that alleges to be a translation of the “lost” Book of Jasher by Alcuin, an eighth-century English scholar. There is also a more recent book titled “The Book of Jashar” by science fiction and fantasy writer Benjamin Rosenbaum. This book is a complete work of fiction.
Another book by this same name, called by many “Pseudo-Jasher,” while written in Hebrew, is also not the “Book of Jasher” mentioned in Scripture. It is a book of Jewish legends from the creation to the conquest of Canaan under Joshua, but scholars hold that it did not exist before A.D. 1625. In addition, there are several other theological works by Jewish rabbis and scholars called “Sefer ha Yashar,” but none of these claim to be the original Book of Jasher.
In the end, we must conclude that the Book of Jasher mentioned in the Bible was lost and has not survived to modern times. All we really know about it is found in the two Scripture quotations mentioned earlier. The other books by that title are mere fictions or Jewish moral treatises.