LONGSHOT By Dick Francis Putnam. 320 pp. $19.95

THERE ARE two kinds of readers: those who like heroes and those who like anti-heroes. The hero, in general, is unswervingly honorable, unquestionably decent, unabashedly straight. Ambiguity never pokes a finger in his eye. And unlike his opposite, the anti-hero, he doesn't seem to stumble onto paths of virtue by way of an accidental detour in the existential maze. He's there because it's simply in his bones to be there.

And there, in a nutshell, is the charm (or, depending on your attitude, the drawback) of the Dick Francis hero -- the man who keeps appearing, under various names and selected occupations, in Francis's intelligent and well-crafted books.

Longshot, his 29th mystery-thriller, sets writer John Kendall, another of his pleasantly engaging young men, on an R-rated version of a boys' adventure plus country-house murder. The result is a thoroughly appealing whodunit, the kind to go to bed with on a cold winter's night or to take -- along with chicken soup, aspirin and honey -- at the first signs of flu.

In fact, so adept is Francis at his craft, so properly insistent on the borders of his isolated, civilized world, that even those readers who prefer sterner stuff may find themselves in sudden agreement with the hero: "Though . . . it was odd to find myself living in the lives of all these people, as if I'd stepped into a play that was already in progress and been given a walk-on part in the action . . . I felt drawn in and interested and unwilling to miss any scene."

Kendall, 32, is a between-novels writer who's been alternately starving and freezing in a garret. Author of half a dozen guides to survival (personally researched in the jungles and the wilds), he's tried his hand at fiction and been struck with its rewards: publication, poverty, solitude and dread. Surviving as a writer, he's learned, is as difficult as camping in a swamp.

And so, when he's offered a chance to make some money by writing the biography of one Tremayne Vickers, a man who's convinced he's led a fascinating life ("Childhood . . . growing up . . . success . . . My life had been interesting, dammit"), well, it's an offer not easy to refuse. Especially when it's sweetened by the heartwarming (anyway, bone-warming) promise of a month in the country at the Vickers estate.

Vickers (are you waiting for the horses?) is a trainer. His family is charming, his friends are attractive and his world is an oyster -- if you don't count the undercurrents roiling all around. MURDER, for one thing. Mayhem, for another. And suddenly Kendall's in the trickiest position of his short happy life.

Francis, whose research is always impeccable, has previously offered us the wine world (in Proof) the gem world (in Straight) and the art world (in In the Frame). This time he offers us the world of survival.

In Longshot you can learn how to build your own fire, how to clot your own blood ("Apply cobwebs to the wound. They're . . . as sterile as most bandages"), how to use your watch as a compass ("Point the hour hand at the sun, then halfway between the hand and twelve o'clock is the north-south line").

Obviously, a set-up like that demands a payoff. Something like the hero getting trapped in the forest with a two- (or is it three-) time killer on his trail, and finding rare opportunity to practice what he's preached.

I have only one quarrel to pick with Francis, though I understand his problem: How do you write dialogue for tough modern characters and still keep it clean?

Francis's solution is to aim for a compromise, with dialogue that's, well, half-buttocked, shall we say. A kind of exercise in curses that never got cursed.

One of his characters is even self-censored, substituting words like "expletive," "bleep" and "deleted" for the "truly offensive obscene words" he means. Some examples: " 'Bleep the lawyer,' Lewis said." and " 'Nolan doesn't expletive like you, dear heart.' "

As I say, I'm completely sympathetic to the problem, which is definitely thorny. But then, on the other hand, it's one of those situations where you're bleeped if you do and deleted if you don't. Linda Stewart writes crime novels and teaches detective fiction.