The Castle of Llyr (The Chronicles of Prydain) by Lloyd Alexander
This is the third book of Prydain and I have to say that it is my least favorite. That is not to say that I don't still love it, but I don't care for it as much as the other four. In this book Taran has a new challenge to overcome, his feelings for the princess Eilonwy. The book is a pretty easy read and it shows another side to Taran, but I feel that it's main usefulness is in setting up Taran's motivation in the next book.
The princess Eilonwy doesn't really act like a princess. For this reason Dallben is sending her to another kingdom to be trained as a "young lady" which is not to her liking at all. Taran and the faithful Gurgi travel with the princess, but when they discover a plot that could endanger her life they must remain until the threat has ended.