A Lonely Death by Charles Todd
The time period is what makes this British police procedural so unique.  It is just after WWI and Scotland Yard detective Ian Rutledge is a former officer of that brutal war.  He is a smart and driven detective, but a troubled man as well.  The trenches are never far from his mind.  To work through details of the case and his life he has regular conversations with Hamish, a man who died under his command, a man who only Rutledge (obviously) can see and hear.  Rutledge is haunted daily by the war in every sense of the word.  
Rutledge is sent to Sussex to investigate the garroting death of three war veterans.  Why are these young men being targeted?  Is it because of their service?  Will more victims follow?
A great pick on audio since the characters and their differing classes are distinguishable by the change of accents.  And Hamish, with a Scottish accent, really stands out.