“the book of Jashar.” This is spelled “book of Jasher” in the King James Version. The word “Jashar” means “upright, straight, correct,” and thus perhaps it should be translated rather than transliterated, and read, “The scroll of the Upright” (“books” as we know them did not exist until after the time of Christ). The “book of Jashar” also occurs in 2 Samuel 1:18.
It is a lost work, but the way it is referred to makes it seem that it was likely some kind of epic poem or record of Israel’s history. The most common belief among scholars is that the real book of Jashar was composed over time. The fact that some events of the time of Joshua were in it, then much later material from the time of 1 Samuel was also in it indicates that it was a compilation of material, some of which corroborated the biblical account, as we see here and in 2 Samuel 1:18. Exactly what it was and what it covered is unknown because the book has been lost. Sadly, the uncertain nature of the book, and the fact that it was lost, has led to a number of attempts to fake and forge a “Book of Jashar” and publish it. One of the last “Book of Jashar” made was a Jewish publication in very good Hebrew that covered the time from Adam to the Judges. But it was written long after the time of Christ and is certainly not the book of Jashar mentioned in the Old Testament. The fact is the real book of Jashar is lost and so we really do not know much about it.
“in the middle of the sky.” So Joshua stopped the sun in the middle of the day.