ian90,
I see some errors in the book of Jasher for instance:
Jasher 5:18 And Noah was five hundred and two years old when Naamah bare Shem, and the boys grew up and went in the ways of the Lord, in all that Methuselah and Noah their father taught them.
Genesis 5:32 And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth.
In Genesis it tells us that Noah was 500 years old when Shem, Ham and Japheth were alive so Noah who was told by God about the future flood upon the whole world, Noah was at the age of 480 years...so Noah was married and had these three sons from the age of 480 up to 500 years old, it never records anything about the age of 502 years old in Genesis as it does in the book of Jasher. Also how do we truly know that Naamah was Noah's wife, Noah's wife name is never mentioned in the bible at all.
Genesis 5:30 And Lamech lived after he begat Noah five hundred ninety and five years, and begat sons and daughters.
You see Lamech had other sons and daughters besides Naamah where it records in Gen 4:22...so their is a possibilty of Noah marry another woman besides Naamah..and these sons and daughters of Lamech probably are his grandchildren or great grandchildren.
There is another error in Jasher:
Jasher 5:20 And all the days of Lamech were seven hundred and seventy years, and he died. 770 years old
Genesis records that Lamech live to be 777 years old.
Genesis 5:31 And all the days of Lamech were seven hundred seventy and seven years: and he died.
I truly believe that God will not let any errors slip into his word and try to make an excuse for Jasher such as Jasher was too tired and forgot to write the additional seven so that would equal out to 777 rather than 770 as it is record in the book of Jasher.
I would not trust the book of Jasher to be the word of God at all.