Peril by Post. John Pickett Mysteries, #8

Honeymoon from Hades... The eighth book in the series finds John Pickett bound for England's scenic Lake District, where an unsigned letter has summoned him for an unspecified reason. Pickett and his wife Julia, the former Lady Fieldhurst, take a room at the Hart and Hound as the letter instructs him. Once installed there, however, Pickett can do nothing but wait for the anonymous contact to identify himself.
A midnight search of the inn's register seems to identify the innkeeper, Ned Hawkins, as his man, but before Pickett can discover the reason for his summons, Hawkins is pushed from a cliff--surviving the fall only long enough to call Pickett's attention to the letter in his pocket. With his contact dead and his only clue a letter containing nothing more than a rambling account of family news, Pickett knows he's on his own.
But that isn't the worst of his problems: Julia saw the crime being committed, and although she can't identify the murderer, there is every indication that the killer knows he was seen--and intends to eliminate any possible witnesses. Amidst a quirky cast of characters including a host of holiday-makers, a bucolic love triangle, an aspiring poet in the Romantic vein, and an old friend of his magistrate, Mr.
Colquhoun, Pickett must discover the secret behind that urgent summons before a second, and far more personal, murder is committed.