Prince Caspian: The Chronicles of Narnia

Peter, Susan, Edmund, and Lucy are returning to boarding school when they are summoned from the dreary train station (by Susan's own magic horn) to return to the land of Narnia, the land where they had ruled as kings and queens and where their help is desperately needed.

This was the second book written in The Chronicles of Narnia. It now stands as the fourth book in the series.