Diana Wynne Jones: Deep Secret

I finished reading Deep Secret last night. My first surprise was that this wasn't a children's book. Young adult, probably, but certainly not the fairy-tale material I was used to reading from her.
The setting is our world, except that there are some people who can use magic and travel to alternate universes. As typical, many of these universes have various creatures and more magic and interact with each other, while the Earth, being a fairly rational and magic-less place, is better not knowing about any of this.
The culmination of the book happens at a Science Fiction / Fantasy convention, and that's what makes this book so entertaining for those of us who have been to these; it's obvious that Ms. Jones has visited one and was amused by her time at it. The people, events and organization are perhaps the most accurate depiction of a convention I've read. Adding real magic to this could be a cliche, but Jones pulls it off with aplomb.
That being said, I don't think this is one of Jones' best books, and the characters didn't have quite the depth I might have hoped for, but that's all quibbling. Also, while the characters may not be as deep as I'd hoped, she does a very good job at telling the tale from two quite different perspectives, painting the exactly same events with two entirely different interpretations.
It was a very fun read and one that kept me up way later than common sense would suggest.

There's another review with some funny anecdotes and more of a description of the plot from Dave Langford, if you're interested.