The Book of Jasher is mentioned in the Book of Joshua and in 2 Samuel 1:18-27. Many believe the Book of Jasher was inserted because Joshua wanted to reference other sources to authenticate the event. The Book of Jasher is one of the apocalyptic Books. The Book of Jasher was left out of the canon of Scripture, however God directed the authors of Scripture to use passages from many extra-Biblical sources in composing His Word. The Book of Jasher contains 91 chapters. Thankyou. James and Hamsa Sasse. GodWhoisGod.com