I'm glad to say that Which Witch is one of my all time favorite books.  It's mostly about a family who has a little boy and once they find out that he is an evil wizard, they try to make him the best evil wizard ever.
     Now that surprised me because I thought that they would probably get rid of him.  Anyway, their son grows up to be Arriman the awful, the wizard of the North.  After ruling for a few years, he finds he needs a wife.  So Arriman holds a contest for all witches who would like to like to be his wife.  Now since he is a famous wizard, lots of witches enter.  If you want to find out which witch Arriman chooses to be his wife, I highly recommend this book to you.