MR. WAKEFIELD'S CRUSADE. By Bernice Rubens Delacorte. 190 pp. $15.95.

THE LONELY, half-mad, hypersensitive narrator of Bernice Rubens' 13th novel sets out to solve the mystery of a stranger and ends by solving his own. A curious strategy (and a successful one), but then Rubens, who was born in Wales of Russian-Jewish immigrant parents, has a penchant for the curious and the circuitous, as well as for the dim corners of the psyche that resist explanation but provide motives.

In Sunday Best she wrote about a family man who leads a double life as a transvestite, and in I Sent a Letter to My Love, about an amorous correspondence between a sister and brother, begun innocently through a personals column. (The latter novel was dramatized and performed in New Haven and London, then made into a film with Simone Signoret.)

Imprisoning solitude; the ambiguities of identity and sexuality; lives precariously balanced on the emotional edges: themes -- as well as the dubious import of letters -- clearly fascinate Rubens. And she handles them with the savage scrutiny of character, the rather cruel humor, and the pathos of the Russian novelists, somewhat diluted and strained through a British accent but potent nonetheless.

LUKE WAKEFIELD filches a letter left by an attractive, youngish man who drops dead before his eyes in the Hampstead post office in London. He steams it open and reads of a murder. In time he stealthily acquires a whole packet of letters exchanged between the dead stranger, Sebastian Firbank, and his presumed wife and murder victim, Marion. Luke sets out to discover Marion's body and inform her next-of-kin, not so much out of moral rectitude, but because Sebastian intrigues him and because he has nothing better to do. An inherited fortune -- through a fairy-tale device -- enables him to live in luxury, overlooking Regents Park, and in idleness, his pursuits narrowed to reading the obituaries in The Times, drinking, and visiting the post office weekly for human contact. The reader becomes his confidant in the crusade springing from one such lucky visit.

But the murder-mystery is simply the piece of meat thrown to the watch dog -- to use T.S. Eliot's metaphor for the writer 's tactics -- to keep it occupied as the burglar goes about his primary business. While the reader eagerly follows the cleverly timed false leads, the dangers and elaborate convolutions of the plot, Bernice Rubens is busy with more important matters. She is offering a painstaking study of a paranoid, compulsive personality -- in Luke's own words, "the master of illusion, practically the inventor of self-deceit"; she is charting Luke's labyrinthine journey to the secret self he has never dared confront; and she is, ultimately, writing about forgiveness, acceptance, and redemption.

Mr. Wakefield's Crusade falls into the genre of the one-man show, a risky enterprise. The sole character relentlessly explored is the son of stolid, unhappy working- class parents who had hoped for a girl. A self-declared failure in work, love, and most of the lesser endeavors, Luke Wakefield has yet managed to become a connoisseur of food and wine, and to "tailor (himself) in syntax and style.' ' Conveniently so: he can mix easily with the upper-crust crowd involved in the mystery. And for the most part he tells his story stylishly: two lapses are the heavy -handed, fairly sadistic attempts at drollery, and some less than elegant repetition, underestimating a reader's ability to recall and interpret the plot.

Luke' s life "has lurched from one catastrophe to another." His brief marriage was dismal, he confesses. When his wife Connie left him for a woman and Australia, he "felt weigheddown with the responsibility of having failed my entire gender." His futile rage against Connie can inflate to encompass all women. Indeed, during the course of his story he manages to knock two down "accidentially," as well as to eject the reappearing and pregnant (artifically inseminated) Connie from a chair, inducing labor and a son he wishes were his own. Graceless, friendless, aimless, Luke is hardly appealing, but almost never dull.

It might seem anomalous that he enlists himself on behalf of poor, abused Marion Firbank, according to the crucial letters a Welsh village girl whom Sebastian found and educated with a Henry Higgins zeal, then tired of when she got fat, i.e., pregnant. Actually it is Sebastian and the elusive friends surrounding him who draw Luke. Even more, the crusade is a mode of escape from his tortured broodings and empty life, walled in by frustration. For a man who can live authentically only in fantasy, what better diversion than a murder mystery germinated in his wn head from puzzling evidence?

Rubens illuminates the dynamics of the fantasy life superbly, and conjures up a surprising denouement. As the mystery is laid bare, Luke finds his way to a life of connection, in which fantasy and reality can fruitfully merge, to dissipate rage. The only structural flaw in this finely orchestrated, engrossing novel is its rush to the finish in too cavalier a manner, unworthy of the rest. The narrative feels summarily dispensed with when Luke finds happiness.