The Book of Jasher is a secular history covering a period from the creation until the period of the Judges in Israel, and a work that is referenced in the Holy Bible twice by two different authors. Three purported copies of this 'Lost Book' have been translated, but only one is looked upon as authentic. The Prophet Joseph Smith quoted from it in Times and Seasons as a source which had "not been disproved as a bad author." Certain members of The Church of Jesus Christ of Latter-day Saints secured the copyright and republished the work in 1887 in Salt Lake City.
The Book of Jasher is almost certainly authentic, as it contains many specific details which are in agreement with modern revelation (i.e. the Book of Moses) that was published before the Book of Jasher became available on the American continent). However, it is also almost certain that it contains modifications from its original form. When studying it, the Lord's advice in Doctrine and Covenants 91 still stands.
Verily, thus saith the Lord unto you concerning the Apocrypha — There are many things contained therein that are true, and it is mostly translated correctly; There are many things contained therein that are not true, which are interpolations by the hands of men. Verily, I say unto you, that it is not needful that the Apocrypha should be translated. Therefore, whoso readeth it, let him understand, for the Spirit manifesteth truth; And whoso is enlightened by the Spirit shall obtain benefit And whoso receiveth not by the Spirit, cannot be benefited. Therefore it is not needful that it should be translated. Amen.