HELP THE POOR STRUGGLER. By Martha Grimes. Little, Brown. 225 pp. $15.95.

IT IS no new thing for British crime writers to set their stories in the States. They've been doing it, from James Hadley Chase upwards, for years. Probably they feel that, because it's a new country, it is also a simple one, that its class system is too clear-cut to pose any problems, and that American society is pretty much the same from coast to coast. In all these assumptions they are presumably wrong, but if they gain confidence from them, all well and good. Many of them hedge their bets anyway by setting their stories among hoods and gangsters, which are as much terra incognita to the average American as to the average British reader.

American writers, on the other hand, have not been so brave, or so foolhardy, as to reverse the process. No doubt they have felt that English society was too complex, too subtle, too riddled with unspoken taboos and imperatives. Added to this, the best-known British crime writers, unlike Hammett or Chandler, have been so damned productive. Who would buy an imitation, when he can get the genuine article at the same price?

So, if we discount exiles like the excellent Marian Babson and Paula Gosling, Martha Grimes is one of the first Americans (someone will immediately shout names at me if I say actually the first) to use England regularly and exclusively as her setting. A lady of great confidence, then. Not only that, one of her detectives, Melrose Plant (is there a half-memory here of John Plant, detective- story writer and hero of Waugh's Work Suspended, combined with Mrs. Melrose Ape in Vile Bodies, who could have contributed hints to his Aunt Agatha?) has had, and renounced, a peerage. That, perhaps, was less than wise. Noble detectives have fallen out of favor since Lord Peter Wimsey, and the choice laid in Professor Grimes' path a string of possible pitfalls about English titles.

Into several of which she duly falls. Indeed, there is the temptation for the English reader to go through her books simply for the pleasure of catching her out; skunks do not exist in Britain (outside the Palace of Westminster); the Left Luggage office at a railway station should not be called the Lost Luggage office (though sometimes it amounts to the same thing). Most stomach- turning are Martha Grimes' versions of the English breakfast: many people believe that the English working classes eat chips with everything; she seems to think the upper classes eat kippers with everything. Most damaging, though, in her early books, are not such slips as these, but a definite feeling throughout that everything is that slight but disconcerting bit off-key, out of focus.

NOW HERE is her seventh, and it is a pleasure to record (having missed heore recent books) that all this is changed. There are still mistakes, often in dialogue ("I guess," "candy," "pretty dumb"), but the feel is now right, the narrative confident and convincing. The pub which gives the title to the book (Grimes's pubs always have names much more interesting than any I seem to go into) is a bleak, uninviting place on Dartmoor, with a hideous crone behind the bar and old Elvis numbers perpetually belted out from the jukebox. We are miles away from quaintness, and the story is a bare, hard narrative of child murder, linked with a murder 20 years before for which the wrong man has served a life sentence.

The best thing about the early books was a Sayers-like boisterous humor, and it is odd to find that growing seriousness suits Grimes, as it never did Sayers. Where Sayers became dull, Grimes takes on a new tautness and purpose. Her middle-and lower-class characters have a ring of conviction, often awful conviction, and her embittered cop who shares the detection with Jury and Plant packs a powerful literary punch. Humor is sparing, charm is miles away. Grimes is often good on inadequate parents and guardians, and this story gives her plenty of opportunities in that line.

There is something of a falling-off in the second half. Here we find ourselves once more with a much-betitled family, and though the characters themselves are well enough done, they become embroiled in a plot that is increasingly unlikely, culminating in a solution that is frankly preposterous. A touch of hokum may be allowable in the comedy-of-manners-style detective story. It is very out of place in what had started out as a very grim product indeed.

But the novel is never less than readable, and much of it is brilliantly successful. One hopes that Martha Grimes' readership will grow so large that she can take a sabbatical year or two in Britain, bury herself in Little- Crabtree-in-the-Mire, and add that top layer of total confidence to her picture of British life.