The fourth of Lovesey's contemporary Peter Diamond procedurals (The Summons, 1995, etc.)--a series that has added substantially to his collection of awards--is cast in the form of a homage to mystery fans. In bustling Bath, a small band of eccentrics gather in a church crypt to argue the virtues of the various genres of crime fiction. As Golden Age devotees face off with noir adherents, local Detective Superintendent Diamond and his colleagues begin receiving rhymed notes threatening theft. And, indeed, a priceless stamp is taken from the Postal Museum; just as the mystery aficionados are considering varying their program to discuss the case, the stamp appears in one of the participants John Dickson Carr paperback! Sooner than you can say ``locked room,'' the body of another participant is found in a locked--um--boat. The formal interviews of the Bath police impinge piquantly on the private gossip and even more private weaknesses--adultery, drunkenness, poor taste--of this group of colorfully drawn obsessives. Lovesey, always something of a Golden Age writer out of his time, provides some ingenious variations on the old 'locked room' mystery formula while gleefully lecturing the reader on genre lore. Still, the author violates the prewar code among mystery writers that protected the reader, handling some of his more sympathetic characters with absent-minded brutality. Adequate Lovesey, then, but hardly destined to be a favorite.