It's a mystery novel by Dorothy Sayers. Lord Peter Wimsey is the hero. It's set in and around an ancient church in the flat and marshy fen country of England (that's the area in the northeast, more or less in Lincolnshire and Norfolk, surrounding the Wash, an inlet of the North Sea).
The murderer is rather surprising.
Many people think it's Sayers's best book, and I tend to agree. It bears re-reading, which isn't true for many mystery novels.
Very sorry to disagree with you but Sayers best book(s) are her translation of Dante's Divine Comedy.
I reread Purgatory at least once a year.