Stardust

A girl tells a boy she will give him anything he wants if he finds the fallen star. He goes on a quest to find it. He enters a magical world of fairyland and eventually finds that the star is a girl. In the process he has all sorts of magical adventures.

We returns and finds the girl is engaged - and in fact had been so since their initial conversation. She had thought she had led him on a futile quest. However, she has not been married, and saved herself to marry him since he had completed the quest. He contemplated the situation and decided that since she had promised him what he wanted, he would like her to marry the other guy.

He instead, marries the fallen star, and discovers that he is also only half human (and half magical). Due to other deaths, they now had the right to rule fairyland. However, they left a regent in place (who they had helped free in the adventures), while they embarked on a large honeymoon tour. When returned, they decided she was doing a good job, and let her continue to rule while the continued exploring.

This is a fantasy book and really hard to get in to.