IF, in its infancy, the mystery seemed thin, all plot and little else, the form has plumped out nicely, particularly in the post-Chandler era, and given us rumpled heroes and strong villains we can really believe in. Now, too, setting has come to play a more important role than ever before. We find private eyes popping up in unlikely places like Montana, and small-town sheriffs sleuthing with the best in the vastness of West Texas. Detectives detect in cities as small as Springfield, Illinois, and Rocksburg, Pennsylvania. So why not Washington, D.C.?

Well, why not? We've had hardnosed cops and gifted amateurs doing their stuff inside the Beltway. Now comes one William Moore, who in his debut novel, The Last Surprise (St. Martin's, $18.95), introduces a singular private eye named Russell McGarvey, who manages to acquit himself admirably in a crowded field. McGarvey, a Vietnam vet and a law school dropout, behaves believably as he rushes around town trying to handle two other cases until he drops everything and devotes his full attention to the big one.

And it is very big indeed, involving the murder of a United States senator in an apparent mugging in downtown D.C. McGarvey is engaged by the senator's wife, who believes her husband was murdered in order to stop an investigation he had underway that would have led to a second Koreagate. His friend on the force, Lt. Pearson, collaborates with astonishing closeness on the case, going over the line a couple of times. Later we find that McGarvey has something on him -- though it's not set forth as crudely as that. There are suspects in government, foreign intrigue and a mafia connection, all of which may make it sound as though Moore has resorted to kitchen-sink plotting. But it is worked out in orderly fashion, right down to his vaunted last surprise (not really a shocker), and the tale is told well in the first-person by McGarry/Moore.

There are details that disturb: in particular, the relationship between the slain senator and a congressman from his state doesn't quite ring true. But what gives The Last Surprise a strong sense of authenticity is the way it has been set so solidly in Washington. It's as specific as a street map, with action played out in restaurants, hotels and on streetcorners that Washingtonians will recognize. Not only has Moore got the city right, but when he whisks McGarvey off to Annapolis for a little R and R with his girlfriend, he gets that right, too. The guy knows his way around. A Capital Crime THAT sense of being there is exactly what is missing from Washington Deceased (St. Martin's, $15.95). It's a sort of Washington insider novel-cum-locked room mystery by a lawyer named Michael Bowen from way out in Milwaukee (such chutzpah, right?). But stifle those snickers! Bowen, who has a couple of earlier books to his credit -- Can't Miss and Badger Game -- writes with such swift wit and cynical humor that you are sure to be entertained, if not entirely convinced, by the activities, comments and cogitations of his amateur detective, Richard Michaelson.

Michaelson is a Foreign Service veteran, now withdrawn to the Brookings Institute to wait for a change in administration so that he may assume what he deems to be his rightful place in the policy-making establishment. Michaelson is asked for help by the daughter of a senator from the Midwest who was caught with his hand in the cookie jar and is now coming up for parole from one of those so-called country-club prisons. But the former senator finds himself a suspect in the murder of a mafia capo that occurs right there in Honor Cottage B-4 in front of the video surveillance cameras. Can the cameras lie? Well, eventually Michaelson proves, with impeccable logic, that they can be fooled.

Although Bowen doesn't seem to know much of Washington outside the Dupont Circle area (where much of the book is set), he does know how to tell a story. Washington Deceased moves quickly, pausing just long enough every couple of pages or so to hand the reader a laugh. And who could object to that? The Gentle Art of Murder IS IT possible to provide too much detail? To tell us more than we want to know about characters or setting or some specific aspect of the plot? If so, Jan Adkins may have done just that in Deadline for Final Art (Walker, $8.95), the first of an intended series of mysteries featuring one Charlie Salt. Or perhaps it's just that there may not be enough plot here to hold together all the other elements in the book -- elements that he handles very well indeed.

Charlie Salt himself is a first-rate character -- three-dimensional, appealing, intelligent, a widower who loves his kids and takes good care of them. His Russian pal, Nikolai Tsiokolvsky, who turns out to be a KGB man (a post-perestroika good guy), is also far from boring. Even the D.C. detective, Inspector Deauville Carlton, a perfunctory role, has shape and substance.

With these and others of nearly equal interest, and action set believably inside the District, Deadline for Final Art really deserves more than the bad old CIA as a threat. It seems that Charlie is an art director at National Geographic, the kind who designs those spectacular layouts that explain it all to the reader. (There seems entirely too much about the inner workings of that magazine included here.) Charlie has reasoned his way to an important and super-secret link in the Star Wars missile shield and is therefore marked for termination by a cell of Company extremists. This premise might have worked a bit better than it does with a few more twists and turns -- more plot. Even so, Charlie, with all his emotional turmoil, is so likeable that he very nearly carries this one on his shoulders alone. He will be welcome back again. Paint It Blackmail IN A WAY, there's not much to say about Rex Burns's Parts Unknown (Viking, $17.95), except that it's an exemplary piece of work. Burns, a teacher of English at the University of Colorado at Denver, learned his craft writing about detective Gabe Wager of the Denver police department. He's now started a private eye series -- of which Parts Unknown is the second -- set in the same city and featuring Devlin Kirk.

Kirk and his partner, Bunchcroft, are true to form as they attempt to scrounge a living from insurance work. But would they take on charity work quite so readily as they do here? Well, of course, had they not gone looking for Nestor Calamaro at the request of his aunt, Devlin would not have had this story to tell.

Nestor is an illegal alien up from El Salvador. It turns out that he, together with others like him, has been victimized by a countrywoman who keeps them in line by threatening to turn them into Immigration. No surprise here -- an old story. What has happened to Nestor, however, is a surprise, one well researched by Burns and made chillingly convincing here. Let's hope Gabe Wager hasn't left the Denver scene for good. In the meantime, carry on, Kirk! Bruce Cook is the author of many books, including a recent mystery novel, "Mexican Standoff."