This is book 2 (at least by the order I know them in) of The Chronicles of Narnia. I recently reviewed book 1, The Lion, the Witch, and the Wardrobe.
I found this a much weaker book than Lion. Aslan demonstrates that he has divine powers more clearly than he did before, but he capriciously demands that Lucy be able to convince the others that he is there rather than just being visible to them. There is an obvious heavy-handed message about the importance of faith. But I still don't see it as being terribly Christian. (In this volume, for instance, other gods actually appear and do godly stuff in the story.) Still, it's mostly a nice story.
7 out of 10.
**** PLOT SUMMARY -- MASSIVE SPOILERS ****
As the book opens, it's a year after the end of Lion and Peter, Susan, Edmund, and Lucy are about to get on a train to go back to school when magic pulls them to another world. They find themselves on an island that seems deserted. They find the ruin of a castle which they eventually realize is Caer Paravel. Two humans approach in a rowboat with a dwarf bound hand and foot. Susan bounces an arrow off one of the men's helmets, and they flee, leaving the dwarf and their boat. The dwarf tells them the story of Caspian, the rightful crown prince. Caspian was told of Old Narnia, the place we know from the first book, by his nurse, and when his nurse was replaced by a half-dwarf tutor, Master Cornelius, who taught him a little bit more about Old Narnia. Caspian's father was murdered by Caspian's uncle Miraz, who was content to let Caspian remain his heir, but Miraz has just had a son, and Cornelius tips Caspian off so he can escape. Caspian finds some Old Narnians and starts to raise an army, but Miraz's real army is winning the fight. Desperate, Caspian blows Susan's horn, which is why Peter, Susan, Edmund, and Lucy were summoned to Narnia. The four children and Trumpkin the dwarf they rescued try to find their way to Aslan's Howe (the site of the Stone Table), but have trouble. Lucy sees Aslan on the way, but none of the others see him and they won't believe her when she tells them the way Aslan is telling her to go, so they lose a day. Then Lucy meets Aslan again in the night, and Aslan forces Lucy to force the others to go his way. Eventually he shows himself to the others, who are contrite for not believing Lucy. They arrive at the the Howe just as the evil faction of Old Narnians is trying to convince Caspian that they should try to summon the White Witch for help. A fight ensues and the evil ones get killed. Peter challenges Miraz to single combat to settle the issue, and two of Miraz' treacherous councilors trick Miraz into accepting the challenge even though he appears to have the overwhelming force. The traitors kill Miraz in the lists and try to do in Peter as well. Then Aslan shows up with an army of awakened trees, who terrify the human army into chaotic flight and surrender. Aslan confirms Caspian as the rightful king. It turns out that the humans who were running Narnia are descended from earthmen who came through another portal. Aslan gives all of them the choice of accepting the return of Old Narnia or going back to earth. He sends the children back to earth through the same portal (though they end up back on the train platform rather than on the remote island).