As mentioned earlier, First and Second Samuel were originally one book. As it turns out, the way that the two books are divided provides a contrast between two kings, Saul and David. The children of Israel asked for a king, and God gave them one after their own heart. But in David, He gave them a king after HIS own heart. This was not because he was faultless, but when he did sin he returned to God in confession and brokenness (see Psalm 51).
David was an extraordinary man: a shepherd, a musician, a soldier, a true friend, an outcast, a king, a great general, a father, a poet, a sinner - but always a lover of God. He was first made king of part of the Jewish nation, the house of Judah. It would be seven and one half long years of struggle until he was made king over all of Israel at the age of 30.
David's reign has been called Israel's golden age. There was no idolatry, and Israel prospered. Militarily, his army succeeded. Tragically, at the height of his powers, David fell into adultery with the wife of one his bravest warriors, Uriah, and has Uriah killed to cover up his sin.
But no sin is hidden before God, and God exposed David's sin through the prophet Nathan. David repented and was forgiven, but the aftermath of his sin resulted in domestic tragedy, including the rebellion of his own son, Absalom, who temporarily usurped his father's throne.
David managed to leave his mark after his own death in numerous ways. He drew up plans and amassed treasure so his son Solomon could build the great Temple of Solomon. The treasure of his wisdom as a father found its way into the book of Proverbs through his son Solomon. Another treasure he left behind were his Psalms, exquisite poetry describing the landscape of a soul pursuing God through loneliness, despair, anger, and ecstasy. And finally, he established the throne of David, the royal lineage through whom one day David's greatest Son would come, the Messiah Jesus.