This is a re-read of one of my favourite Diana Wynne Jones books. The first time I read it, I'd never been to a science fiction convention. Now I have and there are plenty of familiar things in here.
Rupert Venables is a magid charged with selecting a new magid-in-training after his mentor, Stan dies. Stan (as a ghost) is allowed to help in the selection process. What's a magid? Good question and even having read the book I'm not sure I can tell you succinctly. A magid is a magic user who protects and corrects matters any number of alternate worlds, guided by (sometimes very obliquely) entities 'above' who are supposed to know what's going on in the multiverse and guide it along. Confused? Not surprised, but just go along with it. Rupert is the youngest earth-based magid with only a couple of years' experience. Both his older brothers are magids too, which gives him someone to call on when things get sticky. And they're about to get very sticky very fast. The magids mostly keep magic away from ordinary people and there are 'deep secrets' which are for magids only. Some worlds have more magic than others. Earth is on the negative side. Rupert is designated to look after the Koryfonic Empire which is on the cusp of the magic positive/negative divide, but politically unstable. With the death of the emperor it seems to be open season on his heirs and Rupert is caught up in events there, while at the same time trying to hunt down the potential candidates for the new magid. In the end he hits on a plan to get them to come to him... at a science fiction convention. The only one he tries to keep away is Maree Mallory whom he has already crossed off his list because she's weird and they seem to hate each other on sight. But since the fate lines have all become twisted together, Maree turns up anyway together with her aunt, uncle (a writer) and her cousin, Nick. It all gets very complicated because it turns put that there are other magic users playing with the 'nodes' so the rooms in the hotel are never where they were left. it's a plot that ties events on earth with the problems in the Koryfonic Empire.
There's a lot to like in this book. Rupert is an appealing character, Maree and Nick grow on you, and the twisty plot keeps you on your toes. I didn't notice the first time I read it (I wasn't a writer then) but there's a section where Nick and Maree are on their own doing something extremely perilous while Rupert waits for them to return. They return and the plot continues, but for some reason Ms Jones chooses to tell that segment of the story at the end from Nick's point of view, which seems a bit out of place, though it has a nice little reveal at the end.