The islands of Chaldea / Diana Wynne Jones.

The brand new and final novel from the magical and whimsical pen of 'the Godmother of Fantasy', Diana Wynne Jones; co-authored with her sister Ursula Jones. Aileen was supposed to grow up magical - just like the other women in her family. Unfortunately, she's just found out that the magic seems to have skipped a generation...but that's not her biggest problem right now. In her world, there are four Islands of Chaldea. The largest and most magical island has been cut off from the other three for decades - and is slowly draining the magic from them. But now a prophecy has come to light. Someone from Aileen's island will gather a man from each of the three islands, bring down the magical barrier, and unite them with the fourth island again. And according to the king, that someone is Aileen's Aunt - who insists on dragging Aileen along. AND the boy Aileen is sure she'll marry (one day); AND the local boy with more brawn then brain. Someone seems to want to stop them too...someone with an interest in keeping the Islands apart. But still, with magic on their side, nothing can go wrong. Right?