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.
LINK - http://www.sacred-texts.com/chr/apo/jasher/index.htm