This is number 19 in the Ian Rutledge series. Number 21 came out this year. The mother-son writing team called Charles Todd has developed two well-written series, the other being Bess Crawford.
Both authors found the Great War fascinating, so placed Inspector Rutledge in the war in his memories and nursing sister Bess Crawford in the war for real.
For his part, Rutledge has to drag around the memory of Sergeant Hamish. The guilt of the sergeant’s death keeps Hamish alive and actively participating in the investigations. Rutledge tries not to answer him when others are around, but he does slip.
This story begins when the local vicar dies in a car crash which has some evidence of murder. As the attacks and bodies add up, Rutledge finds himself nearly lost for lack of a one solid fact pointing to a suspect.
I read their first book last year, see here. While it was not as polished as this one, it was a solid story, as is this one. I found in this story a lack of red herrings. I could not see any suspects, let alone the most likely. I only realized who the killer was at the same time Rutledge did, which was at the 75% mark on my Kindle, right where the experts say the final push should occur in all stories.
You will probably figure it out before then; there are some clues.
This book is at least 4.5.