By Ross Thomas

The Mysterious Press. 343 pp. $19.95

In a quarter-century as a writer of brilliant thrillers, Ross Thomas has created as many engaging characters as Walt Disney. A lot of Ross Thomas figures are eccentrics and some of them are loony. They are not the sort of people who inhabit the average detective story.

All this means that Thomas has escaped the iron law that exceptionally successful thriller writers tend to get worse with age because they have an incentive to produce for the sake of producing. If your next book is sure to make you millions -- or even a mere several hundred thousand -- the pressure to write a new book fast, and to make it a lot like your last successful book, is overwhelming.

Thomas's books have always deserved to sell lots of copies, and he's certainly had his share of successes. But his novels have never quite made the "blockbuster" category, which is both a shame and a blessing. It's a shame because more people ought to read him. But just missing the Danielle Steel category has endowed Thomas's work with a steadiness and inventiveness that often eludes people who ply his trade.

Long before the world paid much attention to political consultants, for example, Thomas sent a particularly exotic campaign huckster into Africa for a zany experiment in cultural imperialism ("The Seersucker Whipsaw"). Chubb Dunjee, the ex-congressman who stars in Thomas's "The Mordida Man," is like no ex-congressman you've ever met in fiction. And Artie Wu, the hero of "Chinaman's Chance," is truly one of a kind.

Still, even Thomas has produced perfectly normal characters who are well adjusted to the world of detective fiction and could carry a series if they (and their creator) chose to. McCorkle and Padillo, for example. The two own a bar right here in Washington, and they have been the stars of three of Thomas's previous thrillers, including his first book, "The Cold War Swap," published in the mid-1960s.

In his latest book, "Twilight at Mac's Place," Thomas has brought them back to life, to great effect. Their bar has been doing well all these years, thank you, and it has managed to maintain its character as a real -- though elegant -- saloon despite all the changes downtown. Although Thomas has brought everything and everyone up to date, including McCorkle and Padillo, he has not, thank goodness, turned Mac's Place into a yuppie joint.

After all these years, McCorkle still smokes cigarettes. But he's caved in to the health craze to the point that he chews Nicoret gum as part of his unsuccessful battle against tobacco. Let it be noted: This is the first crime novel in history to demonstrate that chewing Nicoret is a mark of tough worldliness.

There's also a delightful new character, Mac's daughter Erika, a young woman who adds sophistication, sexiness and feminism to the family.

The action revolves around the death of Steadfast Haynes, a former CIA agent who dies in his bed at the Hay-Adams at age 57. Steady, as he's known by everyone, has left a memoir called "Mercenary Calling," which begins with this sentence: "I have led an exceedingly interesting life and, looking back, have no regrets. Or almost none."

Many people want to keep the memoir under wraps for good. Some of them are willing to pay Steady's son Granville a lot of money to ensure that outcome. Others prefer to reach that goal by killing a lot of people. Mac and Padillo, experienced in the ways of spies, are called in to help Granville secure the highest possible price at the lowest possible risk to his own life.

As is always the case in Ross Thomas books, no one ever plays it quite straight. There is, in fact, a Graham Greene ethic at work here: Thomas seems to believe that no one who is not at least partly immoral can ever hope to be moral. Thomas characters, who are sometimes gleefully sinful and sometimes fiendishly so, prove Reinhold Niebuhr's adage that original sin is the only empirically verifiable doctrine of the Christian church.

Still, if there is a moralist here, it's McCorkle, and watching the tolerant McCorkle try to deal with the fact that his daughter has jumped into bed with Granville Haynes is to see truth in another adage to the effect that a moral conservative is a liberal with a daughter just out of college. Still, McCorkle manages with effort to keep his liberalism.

This book, like others Thomas has written, is the best kind of Washington novel. It looks at the city as a real city and not as a collection of backdrops for the disjointed activities of politicians, lobbyists, diplomats and spies. And like the best of the spy writers, Thomas sees espionage as a trade like plumbing, carpentry or playing baseball -- something a lot of different kinds of people do with varying levels of skill and dedication. There's no heroic end-of-the-Cold-War stuff here, just a lot of flawed but interesting people protecting their own interests.

The "twilight" in Thomas's title would suggest to us that this could be the last that we'll see of Padillo and Mac, that Thomas will once again veer off into something new. Still, I think McCorkle and Padillo are worth keeping around a little longer. If Ross Thomas wants to take it easy for a few years and just turn out more books about Mac and his friends, his readers won't suffer a bit. The reviewer is a reporter on the national desk of The Washington Post.