An ancient book of national songs, which most likely contained both religious and secular songs describing great events in the history of the nation. In the OT there are two quotations from this book
Then will Joshua speak to Jehovah in the day Jehovah will give the Amorite before the sons of Israel, and he will say before the eyes of Israel, Sun in Gibeon, be thou silent, and thou Moon, in the valley of Ajalon. And the sun will be silent, and the moon stood, till the people will be avenged upon its enemies. Is it not written upon the book of the upright? and the sun will stand in the half of the heavens, and it hasted not to go down for a whole day.
The beauty of Israel was wounded upon thy heights: how have the powerful fallen Ye shall not announce in Gath, ye shall not announce the good news in the streets of Ashkelon, lest the daughters of the rovers shall rejoice--lest the daughters of the uncircumcised shall exult read more. Mountains of Gilboa, no dew and no rain upon you, and fields of offerings: for there the shield of the powerful was cast away--the shield of Saul as not anointed with oil. From the blood of the wounded and from the fat of the powerful, the bow of Jonathan drew not back in the rear, and the sword of Saul will not turn back empty. Saul and Jonathan beloved and pleasant in their lives, and in their death they were not separated: they were swift above eagles, they were strong above lions. Daughters of Israel weep for Saul, having clothed you with scarlet, with delights; bringing up ornaments of gold upon your garments. How have the powerful fallen in the midst of the battle Jonathan wounded upon the heights! Distress to me for thee, my brother Jonathan: thou wert sweet to me exceedingly: thy love to me was wonderful above the love of women. How have the powerful fallen and the vessels of war perished.