The Whim of the Dragon — Pamela Dean

The Third and Final book of the Secret Country Trilogy!
 
Three things have the power to destroy the Secret Country: the Border Magic, the Crystal of Earth, and the whim of the dragon. The cousins Ted, Laura, Ruth, Patrick, and Ellen have faced the first two; now, summoned back to the Secret Country, they must face the third. The Country's most trusted counselors now know that the five are impostors, somehow thrust into the roles of royalty, but no one knows who has been playing with their destinies. The truth lies with only Chryse, the unicorn, and Belaparthalion, the dragon. But getting to them, and speaking with them, is more complex and dangerous than it seems….
 
“Pamela Dean's Secret Country books are required reading for anyone who loves fantasy. Get them!” -- Will Shetterly, author of Dogland