What is your position on the Book of Jashar? I have seen that some Messianics consider it to be legitimate, perhaps even Scripture. This disturbs me.
There are only two references to a book of Jashar or sefer haYashar in the Tanach (Old Testament):
“So the sun stood still, and the moon stopped, until the nation avenged themselves of their enemies. Is it not written in the book of Jashar? And the sun stopped in the middle of the sky and did not hasten to go down for about a whole day” (Joshua 10:13, NASU).
“[A]nd he told them to teach the sons of Judah the song of the bow; behold, it is written in the book of Jashar” (2 Samuel 1:18, NASU).
There is a wide variance of opinion regarding what this “book of Jashar” is. The Orthodox Jewish ArtScroll Tanach renders these verses with “the Book of the Upright” and “the Book of Uprightness,” respectively, explaining that “The Book of the Upright is the Torah, in which God told Moses (Exodus 34:10) that He would act in an unprecedented manner for the benefit of Israel (Radak),” and “The Torah, which alludes to Judah’s prowess in archery (Genesis 49:8).”
ABD makes some important remarks regarding the various traditions for what the “book of Jashar” is. It first says that it is “A lost source book of early Israelite poetry, quoted in Josh 10:12b–13a (Joshua’s command to the sun and moon) and 2 Sam 1:19–27 (David’s lament for Saul and Jonathan).” Noting on the Hebrew term yashar, its entry states “The term ‘Jashar’ is a common Hebrew word meaning ‘one who [or that which] is straight, honest, just, righteous, upright.’ Thus, it is commonly assumed that the title refers either to the heroic individuals who are the subjects of its contents or perhaps to all Israel as the upright people.” Reflecting on what the “book of Jashar” is considered to be, we see that “The mysterious nature of the Book of Jashar has given rise to false identifications and imitations of the book. The Talmud (Abod. Zar. 25a) homiletically identifies the Book of Jashar with the ‘book of Abraham, Isaac, and Jacob’ (i.e. Genesis), who were ‘upright.’ Certain ancient Jewish commentators considered the title to be a reference to the Torah.”
We believe in the traditional Jewish view that the book of Jashar is a reference to either the Book of Genesis or the entire Torah, that either directly refers to, or prophecies and promises of mighty deeds that relate to the people of Israel or Israel’s Patriarchs.
There are some who claim that there was a book of Jashar that originally existed, and should be considered Scripture. However, there are no original texts of a book of Jashar or proof that such a text ever existed. The same entry in ABD remarks that “An interesting example of a more recent forgery from Christian circles is associated with Alcuin, Bishop of Canterbury (d. a.d. 804), who is said to have discovered it in the city of Gazna on a ‘Pilgrimage into the Holy Land, and Persia.’ First published in 1829, it is reputed to have been the words of ‘Jashar, the son of Caleb’ rediscovered in England in 1721. The Rosicrucian Order published a 5th edition of this particular text in 1953.” It is likely that whatever text of the book of Jashar you see published and touted as being “original” is this text, clearly identified as a “recent forgery from Christian circles.”
 Nosson Scherman and Meir Zlotowitz, eds., ArtScroll Tanach (Brooklyn: Mesorah Publications., 1996), 540.
 Ibid., 724.
 Duane L. Christensen, “Jashar, Book of,” in ABD, 3:647.
 Ibid., 3:647.