THIS exceptionally interesting and entertaining book, its author's 18th, falls somewhere between a mystery and a serious work of fiction. This may seem a difficult balance for Stanley Ellin to attempt, but he carries it off with ease and aplomb; there is sufficient suspense in his story to satisfy the demands of genre fiction -- including a climax that came, to me at least, as a complete surprise -- but it really is incidental to the setting and characters he creates. At this more serious work Ellin, who is often confused with the somewhat better-known Stanley Elkin, is most accomplished.

Only the reader with a hard heart and a singular lack of curiosity will fail to respond to the basic situation that Ellin establishes in the opening pages of Very Old Money. His central characters are Mike and Amy Lloyd, a young couple living in Manhattan -- living in increasingly penurious circumstances because Mike quit his job teaching at a private school over a matter of principle, and Amy soon thereafter became unemployed. Their job prospects are bleak, and made no better by the poisonous replies Mike's former boss gives to reference requests. At last, in genuine desperation, they take the degrading step of applying and being accepted for the positions of private secretary and chauffeur to the Duries, a family of great and ancient wealth.

The terms are generous: $20,000 for Amy, the secretary, and $15,000 for Mike, the chauffeur, plus free living quarters in the Durie mansion, food, insurance and transportation. The question is whether they will be worth the sacrifices the Lloyds must make in order to earn them. In the Durie household, a servant is not a person but a part of the woodwork, an invisible functionary who exists solely to keep the Duries in the style to which they are accustomed. Servants -- 16 on staff, eight in residence -- are referred to only by their last names, are never thanked and rarely acknowledged, are expected to be available at any hour to serve as required.

Amy, who is intelligent, independent and spunky, is able to accept these restrictions in good spirit; she knows that because their cost of living will be so low, "if we bank all we can, we'll wind up with about a hundred thousand dollars in the next five years." Mike, however, is a more prickly and irreverent sort who has great difficulty accepting the terms of servitude, but eventually the aspiring writer in him triumphs. He realizes that he has been "plunked down backstage at a marvellous show," one that could be the material for a novel: "Real flesh and blood people insulated by millions of dollars against the world outside and all the more driven to an intense interplay among each other. Very human people when you see them with their hair down. Chekhovian almost."

Of these people whom the Lloyds have been permitted to observe within the obsessively guarded privacy of their great house on Madison Avenue, none interests them more than Margaret Durie, whom Amy serves as private secretary. She is 70 years old and has been blind for more than half a century; she lost her sight in a terrible fall down a steep stairway connecting the second and third floors of the house. For most of the intervening years she was in a deep depression, mourning the abrupt termination of her promising career as an artist; recently, though, she has begun to go out into the world, "suddenly determined to make the rest of her life worth living." AS PART of this process she develops an interest in Braille, and begins to contribute generous support to a makeshift institute dedicated to propagating it. She also, her blindness notwithstanding, revives her interest in art, especially art with a strongly feminist slant. Over the weeks she slowly draws Amy into both of these preoccupations, involving her -- and ultimately Mike -- in sorties the exact nature of which is kept secret from the other members of the household. Finally she fixes her attention on a specific artist: "A young woman who made something of a succ out of town last year. According to report, she has talent, courage, and a profound concern for the female condition. Vilified by some critics -- male, of course -- she steadfastly answers to her own conscience."

It's here that the mystery is provided, in the combination of Margaret Durie, the Braille enterprise, the needy young artist and the "couple-in-service," Amy and Mike, who end up deeply entwined in all that transpires. But the mystery is essentially incidental to Ellin's principal undertaking, which is to show what life is like inside the closed world of the very rich and how two bright, appealing young people come to adjust to it. He must have done his research carefully, for his depiction ofe Durie household has the exact ring of truth. With the exception of Margaret Durie, his focus is less on the members of the family than the machinery that runs the household and the people who operate it. The vast kitchen and pantry, the stockpile of pattern after pattern of exquisite china, the small army of timorous Irish serving girls, the flotilla of unostentatious automobiles, the separate elevators for service and family -- Ellin describes it all in rich and delicious detail.

At the same time he gives us Amy and Mike Lloyd, outsiders who find that against their will they have been taken into the Durie's extended "family" and have accepted it as their own. The relationship between Amy and Margaret Durie is portrayed with special subtlety, especially as it is slowly transformed from the comfortably formal one of servant and served to the vastly more ambiguous one of servant and . . . well, not exactly friend, but something oddly close to that. As all of this takes place the Lloyds learn some important things about the meaning of the word "servant," and some equally important things about themselves and their real relationship with the Duries.

Very Old Money is not, in purely literary terms, a particularly ambitious novel. Its prose is facile and unobtrusive, its themes are modest and modestly argued, its range is relatively narrow. But it is intelligent, good-humored, thoughtful and sophisticated: adult entertainment, in the best sense of both words.