This is one of the apochrypal Books of Jasher. There are several (as many as five) separate works by this title, all composed much later than Biblical times. This particular one is a translation of a Hebrew book printed in 1613. Sepir Ha Yasher, the Hebrew title of this book, means the ‘Book of the Upright’, or ‘the Upright or Correct Record’. This title was misread as ‘Jasher’, and at some point Jasher was treated as a proper name; however the pronoun ‘the’ (hebrew ‘ha’) never preceeds proper names.
There is also another spurious Book of Jasher, published 1750, in which Jasher is treated as the name of the author.

This text covers much of the same ground as the traditional Mosaic books of the Bible, from the creation of the world to the death of Moses, albeit with several minor variations.