My favourite of the Narnia books as a child, which I think is related to the structure and pacing of the story – there's no slow moments, and the tale switches initially between the experiences of the children and Prince Caspian to gripping effect.
Peter, Edmund, Susan & Lucy are drawn back to Narnia after hundreds of years have passed since the events in The Lion, The Witch, and the Wardrobe – to find Cair Paravel in ruins and Narnia under the control of wicked King Miraz and the Telmarines.
Prince Caspian is the true King, and the story tells of his battle to gain the throne and restore Narnia to the talking beasts, fauns, and other genuine Narnians.
Like all the Narnia stories there are underlying Christian themes, but this is apparent to adult readers rather than children – who will find it an exciting fantasy page turner!