The Book of Jasher is twice mentioned in the Old Testament.
Joshua 10:13 "And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day."
2 Samuel 1:18 "(Also he bade them teach the children of Judah the use of the bow: behold, it is written in the book of Jasher.)"
It is because of these two quotes that we know that the book existed prior to Joshua and 2 Samuel. Jasher is an excellent history. It is not considered inspired, but other than a couple of oddities, which could be due to translation, I do not see any issues with the 1840 version. It is too bad it is not complete. There is another version that is less complete and does have some inconsistencies. Jasher fills in quite a few useful details where the scripture is very brief. It is brief where the scripture is more complete. I would say that Jasher compliments the scripture.
This article would be incomplete if I did not mention that there has been a lot of controversy surrounding the Book of Jasher. There is much more information concerning it on line now than just a few years ago. There also seems to be quite a bit more support for it as well from scholarly sources.
There is a New Testament connection for the Book of Jasher as well. It is not specifically mentioned. However there are two individuals who are mentioned in the following verse and are also mentioned in Jasher. They are only mentioned by name once in Timothy and once in Jasher.
2 Timothy 3:8 "Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith."
Now here is the quote mentioning these two men in Jasher.
Jasher 79:27 "And when they had gone Pharaoh sent for Balaam the magician and to Jannes and Jambres his sons, and to all the magicians and conjurors and counsellors which belonged to the king, and they all came and sat before the king."
Here is one further verse that relates to the quote from Timothy.
Jasher 73:28 And Balaam the magician, when he saw that the city was taken, he opened the gate and he and his two sons and eight brothers fled and returned to Egypt to Pharaoh king of Egypt.
29 They are the sorcerers and magicians who are mentioned in the book of the law, standing against Moses when the Lord brought the plagues upon Egypt.
Here is an important detail about Moses that if more widely known would stop some false doctrines. The same could be said for Abram (Abraham). First is the quote concerning Moses.
Jasher 73:30-36 So Moses took the city by his wisdom, and the children of Cush placed him on the throne instead of Kikianus king of Cush.
31 And they placed the royal crown upon his head, and they gave him for a wife Adoniah the Cushite queen, wife of Kikianus.
32 And Moses feared the Lord God of his fathers, so that he came not to her, nor did he turn his eyes to her.
33 For Moses remembered how Abraham had made his servant Eliezer swear, saying unto him, Thou shalt not take a woman from the daughters of Canaan for my son Isaac.
34 Also what Isaac did when Jacob had fled from his brother, when he commanded him, saying, Thou shalt not take a wife from the daughters of Canaan, nor make alliance with any of the children of Ham.
35 For the Lord our God gave Ham the son of Noah, and his children and all his seed, as slaves to the children of Shem and to the children of Japheth, and unto their seed after them for slaves, forever.
36 Therefore Moses turned not his heart nor his eyes to the wife of Kikianus all the days that he reigned over Cush.
So when Miriam and Aaron spoke against Moses it was wrong because Moses did not have relations with the woman of Cush. The following concerns Abraham and Sarah. Genesis 20:12 says,
"And yet truly she is my sister. She is the daughter of my father, but not the daughter of my mother. And she became my wife."
Abraham knew God's law and knew it was wrong to marry his sister as we use the term. However, the Book of Jasher explains this a little further.
1 "And Haran, the son of Terah, Abram's oldest brother, took a wife in those days.
2 Haran was thirty-nine years old when he took her; and the wife of Haran conceived and bare a son, and he called his name Lot.
3 And she conceived again and bare a daughter, and she called her name Milca; and she again conceived and bare a daughter, and she called her name Sarai.
4 Haran was forty-two years old when he begat Sarai, which was in the tenth year of the life of Abram; and in those days Abram and his mother and nurse went out from the cave, as the king and his subjects had forgotten the affair of Abram.
5 And when Abram came out from the cave, he went to Noah and his son Shem, and he remained with them to learn the instruction of the Lord and his ways, and no man knew where Abram was, and Abram served Noah and Shem his son for a long time.
6 And Abram was in Noah's house thirty-nine years, and Abram knew the Lord from three years old, and he went in the ways of the Lord until the day of his death, as Noah and his son Shem had taught him"
So as you can see Sarah was Abraham's sister, but we would say that Sarah was Abraham's niece. In Genesis 14:12 scripture calls Lot Abraham's (Abram's) brother's son. In Genesis 14:14 scripture calls Lot Abram's brother. This also shows that in scripture the terms brother and sister could include nephews and nieces. You can also see from Jasher that Abraham was taught the Law of God from the time he was a child and as the Bible promises he did not depart from it all his life. There are some who believe that Abraham grew up as an idol worshipper. Terah, Abraham's father was an idol worshipper, not Abraham. One last comment on this. If you know how the Hebrew works for singular and plural read the following verse with and with out the plural for daughter. (Iscah is "one who looks forth" the daughter of Haran, the niece of Abraham, the sister of Lot and Milcah. BDB)
Genesis 11:29 And Abram and Nahor took them wives: the name of Abram's wife was Sarai; and the name of Nahor's wife, Milcah, the daughter(s) of Haran, the father of Milcah, and the father of Iscah.
A few years ago I was told how that the Book of Jasher was not mentioned in the Septuagint. This was used to discredit Jasher. However this is not necessarily true. There is no mention in the Septuagint Joshua 10:13. However it is mentioned in 2 Samuel 1:18 under a different name. Here is the verse from the Charles Thomson Septuagint.
"...Behold it is written in the book [entitled] "Straight.""
Here is the same verse from the Brenton Septuagint.
"Behold, it is written in the book of Right."
There is a footnote in the Brenton on the word "Right".
Gr. straight, or right, sometimes upright, as of a man. Heb. Jasher
As you can see what we are dealing with is a simple translation difference. It is the same as when the King James uses Osee in Romans 9:25. Osee is the result of transliterating from Greek to English of the Hebrew word Hosea. Jasher is the result of transliterating from the Hebrew to English the same word that was transliterated from Greek to English to get Straight or Right. Hope that makes sense, but all we are dealing with is the difference you get when transliterating or translating words between two and three languages. Jasher could have been translated as straight, but it was only transliterated in the King James Version.
The 1840 version is the one you will want as there are others that are not as reliable. As mentioned above there is some controversy concerning the authenticity of the book. All books must be judged by Scripture. I think you will appreciate the 1840 version. You can get the book two ways that I am aware of. One is a free download that when unzipped becomes an HTML document that you can read in your browser. Right click and "Save Target As" or "Save Link As" on this -> Book of Jasher.zip. (Don't left click the link as that does not go anywhere.) The file that will download is called jasher.zip which after being unzipped becomes BookofJasher.htm.
The other way to get it is in book form from Artisan Publishers. It may be available elsewhere? Just make sure you get the 1840 edition.