All over the multiverse the Magids, powerful magicians, are at work to maintain the balance between positive and negative magic, for the good of all.
Rupert Venables is the junior Magid assigned to Earth and to the troublesome planets of the Koyrfonic Empire. When the Emperor dies without a known heir, Rupert is called into service to help prevent the descent of the Empire into chaos. At the same time, the senior Magid on Earth dies, making Rupert a new senior desperately in need of a junior. Rupert thinks his problems are partially solved when he discovers he can meet all five of the potential Magids on Earth by attending one SF convention in England. However, the convention hotel sits on a node, a nexus of the universes. Rupert soon finds that other forces, some of them completely out of control, are there too....
Diana Wynne Jones' "Deep Secret" is classic adult fantasy novel by an award-winning author, back in print