What is the book of Jashur mentioned in the Bible?

The book is mentioned in Joshua 10:12-13 and 2 Samuel 1:18-27 but it's not a part of the canon of scripture.

Bible scholars believe that the Book of Jasher was probably a collection of Hebrew poetry and songs that described things like heroic action in battles. There's an 18th century forgery that pretends to be the original book of Jasher, as well as a science fiction book by the same title. They certainly wouldn't be worth your time if you're using them to help you in your study of the Bible.

There are other books mentioned in the Bible that are not found in the canon of scripture, like the Book of Wars, the Chronicles of David, the book of the Kings of Israel and Judah, and the Book of Gad. The people who wrote the Bible never seemed to consider them to be inspired by God. As Peter says in 2 Peter 1, verses 20 and 21, "Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation of things. For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit." These books were referenced in the Bible, but they were only works of history or of philosophy, not inspired by God. Don't be misled by people who tell you that there's another book, even an ancient one that's not in the Bible, that you need to read to fully understand God.