I Samuel 13-17: David comes on the scene

Its hard to imagine that anyone could ignore God's will after being bestowed a kingdom by God, but that's exactly what Saul does. He gets a bog head, raises an army and goes on the warpath in the name of Israel. This is all fine and dandy except that his head grows too large and he starts demanding burnt offerings and ordering the people around outside of the powers bestowed upon him (as outlined by Samuel). Samuel points out to him that because he has disobeyed God, his kingdom will not reign for a long time, rather it will dissipate. 

After this, a slight feud begins between Saul and his son, Jonathon. Jonathon goes after some of the Philistines and is victorious but did so against his fathers blessing. They both admit that Jonathon will die for his disobedience, but the people speak out and save him. Saul continues on his warring ways.

After seeming to make up with God a little bit, Samuel commissions Saul to slay all of the Amalakites (a sinful people). He is to slay all of them without taking any spoils or showing any mercy for anyone. Saul does slay the people but takes the king (Agag) as a prisoner and takes the best livestock to be offered as burnt offerings to the Lord. This is the last straw as Samuel scolds Saul for disobeying God, telling him he is done as king in the eyes of God. Samuel then goes and slays Agag himself. Samuel then  secretly anoints David (the least of Jesse's sons) in the city of Bethlehem to be the new king (An interesting twist that a new king would secretly come out of Bethlehem, eh?). Torn by demons as God's blessing has left him, Saul's aids suggest music to calm his soul. David ends up being the musician Saul finds to come and soothe him with music when the demons strike.

The next chapter (17) is the David and Goliath story which I will not go into any detail about as everyone should know it. There is a timeline ambiguity here though. During this battle with the Philistines, Jesse's 3 oldest sons are reintroduced as having followed into Saul's army, as if they were never mentioned before (They actually were in the previous chapter when David was anointed). David, despite being Saul's armor bearer in the previous chapter only shows up on the scene because Jesse sent him with food for his brothers. After famously defeating Goliath with a sling and cutting off his head, David is not known by Saul. If he was his musician and armor bearer, wouldn't Saul know who he was? Interestingly enough, in our children's bible, David arriving in Saul's court and the Goliath story are reversed. This is one of those things you don't notice when the bible is read in sections. It is through a thorough read through like we are doing that we actually notice these kinds of things. Any thoughts as to why the timeline confusion would be greatly appreciated.