Prince Caspian

The four children return to Narnia, but don't even recognize it at first because it has changed so much. They slowly discover that time is different in Narnia from their own world and that hundreds of years have past. Narnia is now ruled by a wicked man who has no right to the throne. Their adventures involve meeting the rightful heir and raising up "Old Narnia" behind him.

The author puts you right in middle of a thrilling adventure; made more mysterious because children now are looking upon their previous adventures as "ancient history". For my oldest daughter, reading this in early grade school kindled an interest in history and a reverence for "old things" that has remained quite strong years later.