Magids, book 1: Deep SecretDiana Wynne Jones1997

Magids look after all worlds, steer them towards magic, and keep history happening. But Rupert Venables’ mentor has just died, and as the junior magid on earth he has to find a replacement while also trying to find the lost heir of a collapsing empire, worlds away.
Rupert interweaves the fate lines to get all the candidates together at a sci-fi fantasy convention, and havoc ensues as they all converge on a very strange hotel, where everything is always linked, the walls keep moving, people are trying to kill him, and nothing is as it seems…

Titre original : Magids, book 1: Deep Secret (1997)