The Chronicles of Narnia : Book 2 (published order) or book 4 (chronological order)
In this book the Pevensie children are heading back to school, but while seated waiting for trains to take them in different directions to different schools something happens and they are whisked away to Narnia. Only a year has passed for them, but hundreds of years have gone by in Narnia, and Cair Paravel is almost unrecognisable. As is Narnia. But slowly the Pevensie’s realise where they are and how much time has gone by.
They have been summoned by Prince Caspian, for his uncle has unlawfully taken the throne and will kill Caspian if he can.
This is a book in which a lot of nothing happens. The children discover that hundreds of years have gone by, that Narnia has changed, and are brought up to much of what has been going on. They trek through Narnia, and meet up with Aslan, and have a confrontation with the bad guy. Much like in the first book, although much much different.
It certainly doesn’t have the same appeal as The Lion, the witch and the wardrobe but it isn’t terrible either.