"The Secret of Annexe 3", by Colin Dexter.
The 7th book in the Inspector Morse series. After a New Year celebration in an Oxford hotel, a man is found murdered in his room in the hotel's annexe (hence the title!). An intriguing and complex investigation follows for Morse and Lewis.
Thoroughly enjoyed this one. A seasonal read for the time of year and, as with all the Morse books, an excellent story. One point of interest is that this was the first Morse book to be written after the series started to be televised, and Morse and Lewis have now changed into their "TV" versions. Eg in the earlier books, Lewis is older than Morse; he's now younger! Another point of interest is that this is the third out of the seven books in the series I've so far read in which (no spoilers) the killer is not arrested at the end of the book.