Death is Now My Neighbour by Colin Dexter

As he drove his chief down to Kidlington, Lewis returned the conversation to where it had begun.
"You haven't told me what you think about this fellow Owens - the dead woman's next-door neighbour."
"Death is always the next-door neighbour," said Morse sombrely.
The murder of a young woman ... A cryptic 'seventeenth-century' love poem ... And a photograph of a mystery grey-haired man ...
More than enough to set Chief Inspector E. Morse on the trail of a killer.
And it's a trail that leads him to Lonsdale College, where the contest between Julian Storrs and Dr Denis Cornford for the coveted position of Master is hotting up.
But then Morse faces a greater, far more personal crisis ...

Inspector Morse #12