Tell me what each Book is about.
Um...
LWW - The 4 Pevensie children get into Narnia, save it from the White witch, and so forth...
PC - They come back, at the call of Susan's magical horn, to find that hundreds of years have passed, Narnia is in the hands of the Telmarines, and their help is needed for Prince Caspian (the rightful heir) to get it back from his wicked uncle.
VDT - The two younger Pevensies come back, along with their cousin, and take part in Caspian's voyage to find out what happened to the 7 men who were faithful to his father (and sent away by his wicked uncle.)
SC - Two completely different children get into Narnia, to find that it again needs help, this time finding its Prince Rilian, who is held captive by the Lady of the Green Kirtle. (whom some suspect of being the White Witch all over again...)
HHB - A boy and a girl in neighboring Calormen escape from their unhappy land back to Archenland, along with two talking Narnian horses. They discover that Shasta (the boy) is actually the lost prince of Archenland, and heir to the throne.
MN - Yet another boy and girl discover other worlds, witness the death of one, and the birth of a new world - Narnia...where they receive a special apple that saves the boy's dying mother.
LB - Boy and girl back in Narnia (the ones from the Silver Chair, I think?) help with its final battle, to save it from the Calormenes and an evil demon called Tash. Narnia ends, but all the good find a new Narnia - which is basically Heaven...the Pevensie children, and everyone from the other books, reappear dead, in this Heaven...basically a type of "end of the world."
This is very rough, not very good at all...I hope you didn't want more detail?
EDIT: Skandar Keynes is the actor for Edmund in the Lion, the Witch, and the Wardrobe. They can't use just ANY actors NOW, because they have already made a movie with a certain set of actors. It would be odd to people to see different people in Prince Caspian, acting the same children as the ones in the Lion, the Witch, and the Wardrobe...