Mystery writers are always searching for new exotic milieus where they may reenact the ritual of X deducing that Y was killed by Z. Wherever travel agents can book a tour there have been murders solved. Nor is this quest limited to the ends of the earth. There can also be exoticism of class and profession. Vicariously, through Detective X, one can interrogate the titled rich or gossip with thieves in their own argot.

There is yet another territory the mystery writer can explore, though only a few get to it -- that of moral extremity. One night suppose that the theme of murder and its associated wickedness would automatically spark the moral imagination, but in the classic deductive mystery that rarely happens, for we are permitted to view the characters only through the eyes of the cool, clue-seeking detective or his Watson. The detective himself must evidence some moral complexity, as do Chesterton's Father Brown or Chandler's Marlowe, before the whodunit can achieve that highest and so-elusive goal of genre writing, a sense of reality, of three-dimensionality, of depth.

All this preamble by way of applauding the premise of Peter Dickinson's "One Foot in the Grave." Not only has Dickinson hit upon an exotic locale that is a natural for a mystery -- Flycatchers, a luxury home for the terminally ill -- but he's had the further inspiration of having his detective be a bona fide patient there. Retired detective superintendent Jimmy Pibble (the hero of Dickinson's five earliest books) is in sorry condition -- barely able to walk or dress himself and drifting in and out of mental focus like a faulty television set. An excellent way to complicate the deductive process, but more than that, Pibble's own relation to matters of life and death is called dramatically into queston. Pibble is discovered in the first chapter setting out to perform an auto-euthansia; instead he discovers a corpse. By the time the machineries of the mystery proper are thereby set in motion we are inside the skin of one of the most humanly believable and just plain interesting detectives since Phillip Marlowe.

The cast of characters is slim: the staff of the nursing home, a gangster in hiding, a comic grande dame, another gangster, and the police. The mystery itself is slim in proportion, and its solution rather strains credibility, especially for readers who may take exception to a denouement that relies heavily on the lore of hypnotic power. Readers who boggle at The Moonstone will surely boggle here too, but the book remains a delight for all that. As in most other good mysteries the plot is no more than the thread needed to string together the hero's successive encounters with the spectrum of people around him.

Though these characters are few, they are choice, and keenly observed. Dickinson's tone ranges from the tape cassette realism of George Higgins to the droll exaggerations of Wodehouse, and at neither end of that range does he hit any wrong notes.

Readers like myself who are only now discovering Peter Dickinson will find that the greatest bounty of this book is the "by the same author" list of other titles. My instinct tells me that Dickinson is one of those blessed writers whose every novel is a certain source of pleasure, something to be hoarded against the day that you require an absolute assurance of being intelligently entertained.