ONE OF THE oddities of contemporary culture is that American writers have defaulted, almost to a man, on their obligation to provide readers with witty, literate and intelligent popular fiction. American fiction these days is divided between the devil and the deep blue sea: between the cynical purveyors of exploitation fiction on the one hand, and the ivory-tower academic novelists on the other. There seems hardly any middle left at all. For fiction that means primarily to entertain, but that assumes its audience to be reasonably sophisticated and alert, we must turn to the British, whether for novels of manners (Barbara Pym and Isabel Colegate) or for thrillers (John le Carr,e and Len Deighton). Apart from John D. MacDonald and Elmore Leonard, there's hardly an American writer alive who writes escapist fiction that a person of discrimination can read without wincing.

But there is Ross Thomas, and it is about time that he got the large, enthusiastic readership he deserves. Briarpatch is his 20th book, a well-rounded milestone that, if there is any justice left, should get him over the hump and onto the best- seller lists. Whether writing under his own name or that of the pseudonymous Oliver Bleeck, Thomas consistently turns out novels that, while providing the requisite amounts of suspense to qualify as genre fiction, are distinguished by expert prose, penetrating social commentary and -- bless you, sir -- a marvelous sense of humor.

All of which are abundant in Briarpatch. I can't say that it's the best of Ross Thomas because I haven't read all the others, but it certainly is quintessential Thomas -- and that, in my judgment, is about as high a compliment as a piece of popular fiction could be given. To begin with, it amply fulfills the first requirement: you can't put the damned thing down. Its characters are real, and interesting, and there's not a piece of cardboard in the lot. Its setting -- "the capital of a state located just far enough south and west to make jailhouse chili a revered cultural treasure" -- is depicted with an impressive eye for small but telling detail. And there are enough laughs, including the funniest CPR rescue ever written, to make a happy evening for the speed-reader or a gleeful week for the dawdler.

Among the many pleasures of reading it, one of the most satisfying is simply to watch Thomas juggling -- and never once dropping -- two complicated plots that for over 300 pages seem to have nothing to do with each other except that both involve the central character. His name is Benjamin Dill, he is 38 years old, and he works on Capitol Hill as a $168-a-day consultant to "an obscure three-member Senate subcommittee on investigation and oversight." He is a native of the aforementioned state capital, to which he is drawn back by terrible news: the murder of his 28-year-old sister, a homicide detective in the municipal police department, in a car-bombing. So long as he is going there, and so he can put the trip on his expense account, the minority counsel suggests he interview a former CIA operative whose affairs the subcommittee is looking into.

Thus we have the two mysteries: the murder of Felicity Dill and the tangled past of Jake Spivey. As is always the case in the best suspense fiction, though, there is more to these matters than at first meets the eye. For openers, as Dill begins to look into his sister's life he discovers that there was a great deal about it she never told him: that she'd had a love affair with a former pro-football player turned private eye, for example, but that she'd broken it off for a fellow cop whom, before her death, she had said she would marry. More than that, as Dill observes: "She was leading a pretty strange life before they blew her away. She bought a duplex she hardly lived in with money she didn't have. She took out a two hundred and fifty thousand dollar life insurance policy, paid cash for it, and died three weeks later -- right on schedule. Doesn't anyone wonder. . . where the hell the money was coming from?"

BY THE TIME matters are more or less sewn up just about everyone is asking that question, or trying to avoid answering it, and a great many people are in a great deal of trouble, some of it of the fatal variety. Not merely are there Dill and the woman lawyer he has fallen for and a spaced-out fellow with a genius for electronics and the ex-footballer and a nasty old newspaperman and enough cops to populate "Hill Street Blues"; there are also all those "renegade spooks" whom Dill is trying to track down. Chief among these is Spivey, who was Dill's closest boyhood friend and who remains, their conflicting interests notwithstanding, a person about whom he cares deeply. Spivey made himself a fortune in Vietnam after the war by co-founding with another apostate CIA agent a company "that bought surplus equipment from the new Vietnamese government and sold it on the open market to whoever wanted to buy it" -- equipment along the lines of "defensive weaponry, transportation, communications." Now Spivey is wanted by Dill's subcommittee, either for his own suspected violations of the law or as a witness against even bigger miscreants.

It's a difficult position for Dill, one that forces him, as his lover the lawyer puts it, "to choose between your friend and your government." As much as anything, that's what the novel is about: friendship and loyalty, the ties that bind people to each other and that can make life unbearably complicated when other considerations enter the picture. Whatever Dill's frailties, and they are not inconsiderable, he is a principled man; his deepest loyalties, he discovers, are to his sister and his friend, and it is to the memory of one and the future of the other that he dedicates himself -- although even this dedication must ultimately be compromised, in the higher interests of self-preservation.

This is serious stuff, but Thomas doesn't let himself get too serious about it. One of the most appealing characteristics of his work is that it never lets you get very far from the knowledge that all this foolishness is just that: foolishness. Thomas is always there in the background, though never obtrusively so, making wry, deflating comments on his characters, exaggerating things a bit just to emphasize their silliness, poking fun at the vainglorious and pompous. Certainly there are more certifiably "serious" American writers than he, but there's no one who gives more pleasure to the page. Ross Thomas is a craftsman, a professional in the most admirable sense of the word, and he does what only the best writers can: he leaves you wanting more.