The Secret of Annexe 3

The Secret of Annexe 3 is the seventh novel in the Oxford-set detective series from Colin Dexter. As portrayed by John Thaw in ITV's Inspector Morse.
Morse sought to hide his disappointment. So many people in the Haworth Hotel that fateful 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.
With the corpse still in fancy dress - albeit bloodsoaked - and hardly a single guest at the Hadworth hotel having checked in under their real name, Morse is faced with his toughest mystery yet.
The Secret of Annexe 3 is followed by the eighth Inspector Morse book, The Wench is Dead.