This welcome appearance by Innes' Sir John Appleby--by rough count his 30th outing since arriving on the scene in 1936--is rather morbidly offered as ""what may prove to be Appleby's final performance."" Though now 80, however, Innes--the grand old don of British mystery fiction--seems in fine fettle here, with enough dry wit and relaxed erudition to compensate for a slender, ho-hum plot. Lord Osprey of Clusters (a vast country manse on a bit of island surrounded by muddy water) has been found dead in his library, stabbed in the throat. So retired super-cop Appleby, a neighbor and acquaintance, reluctantly agrees to help in the sleuthing. . .and tidies things up in about 24 hours. At first the likely culprit would seem to be the local tavernkeeper, whose nubile daughter was allegedly ravished by dirty old Lord O. But Appleby focuses instead on Lord O.'s famous coin collection, which is hidden somewhere in the house. And, besides the inevitable butler, the suspects (down for a weekend house-party, of course) include Lord O.'s sullen son, his flaky brother-in-law (a coin scholar), and two ladies with their own numismatic obsessions. The puzzle is paper-thin, nearly transparent; there's virtually no action (aside from a Hitchcockian last-page demise for the killer). But devotees of refined English mystery-comedy will find this a tasty little morsel--with suave repartee (though Appleby's straight-faced use of the ""nigger in the woodpile"" expression should have been edited out), gently chortling narration, and a country house whose odd features include a bona fide ""trompe-l'oeil trompe-l'oeil.