The Princess Who Flew with Dragons
The third book in the Dragon Heart series.
Sofia is the other princess. Not the crown princess - that's her perfect big sister, Katrin. Sofia is the princess who always says the wrong thing; who hates going to parties; who doesn't like people; who just wants to sit and read her books.
So when Sofia is shipped off on a stomach-churning dragon flight to make another royal visit, she decides it's time for this princess to take a little break. In disguise, Sofia soon finds herself discovering a magical city of learning, new friends and freedom. In short, everything is going wonderfully... right up until it goes horribly wrong. And that's before the ice giants show up.