Bury Your Dead by Louise Penny.*
The story begins with what we soon realize is a flashback to an incident that occurred at an unspecified time.
Then, we realize that Detective Inspector Armand Gamache is in Quebec City, visiting an old friend and mentor. His wife, Reine-Marie, has been with him, but returned home to Montreal that morning. Slowly, it is revealed to that Gamache is in the process of recovering from a trauma.
In Quebec City, he is filling his days by doing some historical research at the library of the English Literary and Historical Society. One day,he arrives to see emergency vehicles outside, and is at first not allowed to enter. Then, the Inspector in charge recognizes Gamache, and tells him that a body has been found in the sub-basement of the historic building, and that the body is not a centuries-old skeleton as might be expected, but Augustin Renaud, a local historian.
Gamache agrees to consult with Inspector Langlois on the case, partially to avoid thinking about the incident he's trying to come to terms with, a situation he believes he handled badly.
Gamache receives daily letters from Gabri Dubeau in Three Pines stating that his partner Olivier Brule is innocent of the murder for which he's been convicted (see The Brutal Telling). He begins to doubt hhis handling of Olivier's case as well, and asks his assistant Jean Guy Beauvoir, also on leave, to go to Three Pines and investigate. Though Beauvoir hates Three Pines, he'd do anything for his Chief (and is frankly relieved to be doing something).
The narrative moves easily between the flasbacks which slowly reveal the progress of the incident that put them both on leave, Gamache's investigation in Quebec City and Beauvoir's investigation in Three Pines.
This may well be the best book of a series in which all the books are extraordinary.
*Many thanks to LibraryThing Early Reviewers for the ARC.