The Three Evangelists, by Fred Vargas (1995)
As unlikely as it might sound, Fred Vargas is the writing name of a French female author of detective fiction. This, her first novel, was written in 1995 but not translated into English until last year. I have not read any others (she has written 12) from her (yet, although I plan to): according to Wikipedia, the characters who appear in The Three Evangelists recur in the subsequent novels.
Unlike most policiers, it does not centre upon policemen (or women) as its detectives. Instead, there are three historians (Marc, Matthias and Lucien) and a retired policeman, Armand Vandoosler (godfather of Marc) who tends to refer to the other three as saints, much to their discomfort. Of course, they are historians from three very different eras (a pre-hi storian, a medievalist and a student of the Great War). All are in their mid-30's, very single and "seriously unemployed". Despite the fact that they are each highly disdainful of each other's professional period (Lucien is dismissed as an excitable modernist), they take up residence in a ramshackle old house known as "The Disgrace", with the former cop in the attic.
The initial mystery they're called upon to solve arises when their neighbour, Sophia, wakes up one morning and finds a tree has materialised in her garden overnight. She's worried that something may have been buried under it, and pays our heroes a large sum of money - not so much for the task of digging up the tree as for keeping quiet about whatever it is they might discover.
But the real mystery arises a couple of weeks later, when Sophia herself disappears. Might she be under the tree herself? She's rich, had some fame in the past as a notable opera singer, had become caught up in some sort of vendetta by a couple of critics, had an obsessed fan or two. Our heroes get on the trail, with the husband (as he always is) the prime suspect. The proper police, however, have other ideas: a niece of Sophia has turned up out of nowhere, one who has pulled at the heartstrings of two of our heroes. So, vindicating her becomes just as important as finding the killer.
Like all good murder mysteries, there are several false leads, and knowledge of the killer's identity is withheld until very near the end, at which point there is a mad rush to prevent one of the Evangelists from being killed. As to how the killer was identified, it was by processes which no policeman would employ. Indeed, it had a lot to do with Lucien's mad passion for the Great War: in trying to track down some diaries of that time, he came very close to solving it. Then Marc had a single "tectonic shift" in which all of the clues they'd been given were seen in a new light. As his uncle says "Medieval historians have special ways of thinking. When Marc gets his mind in gear, he gets straight to the answer. he takes it all in, important stuff and rubbish, and then all at once he goes for it." An interesting technique for crime solving.