The Ampersand Papers (1978) - Michael Innes
This is a mystery set at an old Cornish Castle which is the ancestral home of Lord Ampersand. Innes' main detective hero, Sir John Appleby, is in this novel but he is now a retired Scotland Yard investigator. Unfortunately, the crime in the story is a bit of a bore with some missing papers that may or may not have some connection to literary legends (Byron, Shelley, etc.). at the point this book was published, Innes had been writing about Appleby's adventures for nearly 40 years so you can feel the tedium. There is, however, something to be said for the skillful use of setting and the deft use of language by Innes.