58. The Weathermonger by Peter Dickinson (book 3 of the Changes trilogy)

The Weathermonger was actually the first book written.  I haven't looked into it, but I think after its success, Dickinson was encouraged to go back and write some more taking place in this world.  I think in some ways, this is the best of the three.  The story here takes place about a year after Heartsease (about 6 years after the Changes happened).  It starts in medias res with a boy and his younger sister stranded on a little rock while the tide comes in and a gang of villagers are standing on the shore line, rocks in hand, to ensure that they drown.  The boy has no memory of what happened, but the girl slowly fills him (and the reader) in.  He's a weathermonger, a local magician responsible for ensuring good weather for the village.  He got too greedy and they decided to take him down.  He lost his memory due to a blow to the head.  However, losing his memory brought back his understanding of the world before the Changes.  He and his sister then plan to escape.  I won't go into any more plot details beyond saying that they do escape and then are sent back, to try and figure out the source of the Changes. 

The first half of The Weathermonger has a similar trajectory as the first two books in the series, an adventurous journey with children in charge.  It's the second half, when they meet the weathermonger and start to unravel his mystery, that takes this book to a higher plane.  It's fantastical and really enjoyable.  Very cool stuff and I can see now why as a kid I snuck and read ahead to find out what happened.  The whole series is great, but I think the Weathermonger really stands alone as well. If you only want to read one of them, this is the one.