Prince Caspian by C.S. Lewis
Series: The Chronicles of Narnia #4
The Pevensies reenter Narnia in a very different way and arrive to find the world that they love drastically changed. Instead of winter, men have taken over Narnia and the true inhabitants have been forced to hide in dark corners. There is one named Caspian who was heir to the throne but it was stolen and thus with Aslan and the Kings and Queens of ages past to guide them the Narnians take back their country. Even though the children grew to adulthood in Narnia once before there is something about their growth in the outside world that changes their connection to Narnia forever.