Aileen is a young girl and comes from a magical bloodline. It is her destiny to follow her aunt and late mother as a Wise Woman of the Islands of Chaldea. There are four islands to the world, three of them cut off from the largest because of a spell cast a little over a decade before. The feel is generally Celt-ish.
There are shortages of certain goods and some political turmoil because of the division of the islands, but Aileen's biggest worry is that her magic has not revealed itself yet. She is apprenticed to her aunt but she worries she may never be a capable leader. The worries are pushed aside when she finds herself, her aunt, a handsome prince, and an orphan are sent out on a quest to end the division of the islands.
Diana Wynne Jones was a fantastic author and 'Islands of Chaldea' has many elements of her successful stories. We'll never know what the final novel would have looked like, but this was a fun diversion. I'm glad to have read it, but it doesn't stand out of the pack of her other, finer, work.