This is an unknown book that is mentioned in Joshua 10:13 and 2 Samuel 1:18. A copy of this book has never been found, but it is believed to be a book that contains songs and poems about praising heroes of Isreal in battle.

This site has a lot of great information regarding it : https://www.christiancourier.com/articles/1026-what-was-the-book-of-jashar

“The expression, “book of Jashar,” translates two Hebrew words that signify “book of the upright” or “book of the righteous one.” Apparently it was an ancient Hebrew hymnbook containing nationalistic songs.”

Another site with more information: https://www.gotquestions.org/book-of-Jasher.html

 

I began reading the book of Joshua last night and came across the part where the people of Israel are commanded to kill everyone, men, women and even children. Like nearly anyone with a heart, it is hard to for me to understand why God would command such a thing. Even with the understanding that this is a much harsher and violent time period, I still recoiled at such a barbaric activity. Seeking guidance I did some research and found some good explanations by others. 

This is a pretty interesting site and explanation. He basically says “These people were sinners (as all people are) and God did not want them to have temptation around them. Every minute of your life, is granted to you by God, he can take your life when he chooses fit, regardless of who you are. We mere humans are like ants compared to his infinitesimal wisdom, and have no real right to question his authority. That is a hard thought to wrap your head around, but not liking his decision, and choosing to believe he does not exist for that reason, does not change the fact that he is there and very real. http://www.ligonier.org/blog/why-did-god-command-children-israel-kill-every-man-woman-and-child-promised-land/

 Another site that further expands upon this thought process has an excellent passage: 

“With Joshua there was a political, ethnic dimension, God was immediate king, and he uses this people as his instrument to accomplish his judgment in the world at that time. And God, it says, let the sins of the Amorites accumulate for 400 years so that they would be full (Genesis 15:16), and then sends his own people in as instruments of judgment.”  Taken from this site: http://www.desiringgod.org/interviews/what-made-it-ok-for-god-to-kill-women-and-children-in-the-old-testament

Yet another great site that further expands upon this topic. https://bible.org/question/how-could-loving-god-tell-israelites-kill-their-enemies-even-children

He also recommends two books that look very intriguing. What If Jesus Had Never Been Born? and What If the Bible Had Never been Written? by James Kennedy and Jerry Newcombe.

Lastly, and probably the best explanation of the conquests listsed in Joshua is from the site listed below. It explains the extreme immorality of the people of Canaan, and more of the reasons why this order was given. https://www.namb.net/apologetics/joshua-s-conquest-was-it-justified