The Secret of Annexe 3 by Colin Dexter

‘Morse sought to hide his disappointment. So many people in the Haworth Hotel that fatal evening had been wearing some sort of disguise—a change of dress, a change of make-up, a change of partner, a change of attitude, a change of life almost; and the man who had died had been the most consummate artist of them all...’
Chief Inspector Morse seldom allowed himself to be caught up in New Year celebrations. So the murder inquiry in the festive hotel had a certain appeal.
It was a crime worthy of the season.
The corpse was still in fancy dress. And hardly a single guest at the Haworth had registered under a genuine name...