Evidently, the Book of Jashar once existed. It is quoted two places in the OT:

Joshua 10:12-14; "On that occasion, when Yahweh routed the Amorites before the Israelites, Yehoshua addressed Yahweh; he spoke, in the
presence of the israelites: 'Stand still, O sun, at Gibeon, O moon, in the valley of Aijalon! The sun stood still and the moon halted while a nation wreaked judgment on its foes. As it is recorded in the book of Jashar so the sun halted in mid-heaven and did not press on to set. For an entire day Yahweh fought for israel. Not before or since that time has there ever been such a day in which Yahweh acted upon the words spoken by a man.'

2 Samuel 1:17-18; "And David intoned this dirge over Shaul and his son Jonathan: 'He ordered the Judites to be taught the bow.' That is what is recorded in the book of Jashar ."
However, existing copies today are likely frauds:

The original 'book of Jashar' is thought to be an Israelite collection of poems and chronicles, quoted from in various books of the Bible. Most scholars see the work as a collection of oral traditions which were compiled at about the time of David or Solomon in the 10th century B.C. though the origin of this work seems uncertain.

Thought to be lost, some claim the current versions to be forged in the 18th century in an attempt to fill the gap otherwise left in the mentioning's of the above bible books. Reading it one will immediately feel familiar with its content. There exist two ancient rabbinical works and also an anonymous Jewish work of the 12 th. century AD. which are titled 'Book of Jashar'.
The danger with many Apocrypha and 'lost books' are the late date they are written. Not only are they written too late to be considered inspired, but some were authored by full-blown Pharisee-rabbi's. Jashur falls into this latter category, and even if there was the residue of genuine text, it would be impossible to separate from late-rabbi/Pharisee extrapolations. Therefore, Jashar is plainly unreliable.

Christian theologians and teachers, in general, reject the book due to the fact that it might be 'Jewish legendary history', although they do acknowledge that it is written in 'good' Hebrew, but most reject it due to the fact it is of uncertain etymology....

The notorious CSCS website said:
As to the fabrications... there were three medieval attempts, all written by Jews in Hebrew. One is a moral treatise written by the Rabbi Shabbatai Carmuz Levita in 1391....Another is an introduction to the Hexateuch; this was published in Venice, 1625...(the Hexateuch is a copy of the first 6 books of the Bible--Genesis to Joshua. I guess jashur first appeared as a mysterious insertion by an unknown copiest...The 1840 version of jashar is from the 1625.
Here's a Protestant statement regarding the value of Apocrypha:

The Belgic Confession
Article 6: The difference between Canonical and Aprocryphal books

We distinguish between these holy books and the apocryphal ones...The church may certainly read these books and learn from them as far as they agree with the canonical books. But they do not have such power and virtue that one could confirm from their testimony any point of faith or of the Christian religion. Much less can they detract from the authority of the other holy books
Pardon the lengthy disclaimer.... Can anyone debunk the rabbinical 1625 origin of Jashar? Does it matter?
Anyway, here's Jashur's interesting verses. They're in reference to Gen. 34, the race-mixing story of Dinah and the Shechemites...

Jashur 33:21,"And the came back and sat before their father and they spoke unot him kindled with wrath, saying, Surely death is due to this man and to his household, because the Lord God of the whole earth commanded Noah and his children that man shall never rob, nor commit adultery; now behold Shechem has both ravaged and committed fornication with our sister"

33:17-18, "Hamor said unto his son, Shechem, Is there then no woman amongst the daughters of thy people that thou wilt take an Hebrew woman who is not of thy people. And Shechem said to him, Her only must thou get for me, for she is delightful in my sight"

33:34, "And when they had gone, the sons of Jacob said unto their father, saying, Behold, we know that death is due to these wicked ones and to their city, because they transgressed that law which Noah and his children and his seed after them. And also because Shechem did this thing to our sister Dinah in defiling her, for such vileness hsall never be done amongst us"
Given the alleged origin of Jashur (1625 AD not to mention Jewish authorship), I'm not sure what value these verses may have. But, the author claimed Shechem was guilty of two sins--rape and adultery. Since Dinah was unmarried (a young virgin) and polygamy wasn't a crime for men, my only guess is "adultery" meant miscegenation. The heinous crime associated with "the commandments given to Noah"... Jashar seems to suggest the 7th commandment is a prohibition against miscegenation.