The Book of Jasher is referred to in the Bible in Joshua and Second Samuel.
The name Jasher in Hebrew means; the upright or correct record.
The most important value of this book is the large quantity of additional detail
it gives to various accounts in the Old Testament. For instance, the translator
states in his preface:

This book contains a more detailed account of the awful circumstances
attending the commencement of the flood, and of the conduct of Noah toward
the terrified multitude who had assembled about the ark, when the fatal
moment had arrived, and their doom was irrevocably fixed.