The Silver Chair by C.S. Lewis

The Silver Chair by C.S. Lewis is one of my favorite books in the Narnia series.

In it, Eustace Scrubb and Jill Pole are drawn to Narnia to rescue a kidnapped prince. But because of a fear of heights that Jill has, the pair start to muff things up. First, Jill accidentally pushes Eustace off a cliff, then Eustace doesn't recognize an old friend (since time passes different on Narnia than on Earth). And this isn't the first mistake the two friends make! But with a gloomy yet likeable guide and with the help of Aslan, the two friends might just stand a chance of rescuing the prince!