"Service of all the Dead" by Colin Dexter.
The 4th book in the "Inspector Morse" series. We're still in "pre-TV Morse" days here, and the book Morse will be surprisingly different to those who are only familiar with the TV series.
This book has Morse investigating a puzzling series of murders associated with the church of St. Frideswide in Oxford. The usual complex web of theories and counter-theories follow, which are the joy of all the Morse novels, before the solution is finally revealed. Perhaps not quite as surprising as some of the others, but still a truly excellent book which I thoroughly recommend.