Syl, with regard to the Book of Jasher, from the page you linked to..."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'."
From Wikipedia... "The Book of Jasher is the normal English name (used by, e.g., the King James Bible) of a work known in the original Hebrew as Sefer haYashar, (??? ????; also transliterated Seper haiYašar). It is the best-known of several "Lost books of the Old Testament", books referenced in the Hebrew Bible of which no copies are known to exist."