Howard Brooks-Baker has traced his ancestry to the 15th century, wears a coat of arms on a large lapis lazuli ring, married to La Rochefourauld, belongs to the Metropolitan and Chevy Chase Clubs here and to the Turf Club in London and the Jockey Club in Paris, and is anxious to prove that he is Alex Haley's equal.

He, too, he keeps saying, is not a snob, but is interested in his ancestors and in geneology in general because of how it illuminates history and one's place in it.

Brooks-Baker, with some other bankers and Lloyd's underwriters, bought Debrett's Peerage last year, and as a great, non-snobbish gesture (which has turned last year's deficit into a profit) has taken out advertisements offering "the same expertise to investigating the ancestors of redblooded commoners as we have given in the past to blue-blooded aristocrats."

Debrett's is now going in for a number of new enterprises, in addition to taking on the red-blooded. A jet-set social register called the Almanack International will be out next year, listing "well-known families that live in more than one country." A new etiquette book on "the manners of the English-speaking world" is in progress, as are a book on Scottish clans, a book on Queen Victoria's two jubilees and a catalog of the Victoria and Albert Museum's exhibition of Fabrege objects.

Social listings for several American cities, including Washington, are contemplated. "Our original idea was to buy the existing ones and put our name on them, but we found we would have to change them so drastically, that we might as well start from the beginning."

Since the New York office of Debrett's Ancestry Research opened at 1675 York Avenue last Thursday, there have been 900 letters from prospective clients. The London office, opened in February, received 4,300 inquiries, mostly from Americans.

About $285 should get you two or three generations back from your grandparents, whom you probably know. But fees go by time spent, and a family difficult to trace could cost thousands.

Why do people want this? "People feel more and more like numbers, if you know where you come from, you feel more yourself, a real person. It tells you what you are."

Brooks-Baker, whose hyphenated name was made in 1792 when "a poor farmer" named Baker married "an heiress - I mean a small heiress" named Brooks, comes from an English background "with a bit of French, Dutch and Irish" - he describes as "perfectly normal. There were small landowners, big landowners, merchants, schoolteachers - yes, a nobleman.

"Practically everyone can find a nobleman in his ancestry if that's what he wishes, trick is to find how to do it. You're related to everyone essentially, if you go back far enough.

"A serious geneologist could care less if you're related to a dustman or a duke. He's interested in finding out who you are."

Who Brooks-Baker is, more recently, is the son of a Washington lawyer, who got interested in geneology when he was at Landon School for Boys. His early career was spent working in political campaigns for former Gov. Harold Stassen, and in 1958, Brooks-Baker became a banker in Switzerland.

He is now managing director of Debrett's, where he has learned that "most people" are as unsnobbish as himself.

"Most people today would not be elated - or upset - if one of their children married a nobleman - or a grocer. I wouldn't care which my children married, if it were someone I liked. Oh, I suppose the champagne at the wedding would be better with the nobleman.

"But look at the European monarchs - they marry perfectly ordinary people. The Queen of Denmark, the King of Sweden, the Crown Prince of Norway. They all married ordinary people. That would have been unheard of a few years ago."

He has also observed that prejudice has practically disappeared. "When you show someone - as we do every day - illegitimacy in his family, and every racial origin, they're delighted. The stranger, the more exotic, the better. Some people may still have prejudices, but they're in a minority. This provincial side of life is coming to an end.

"Last night at the Chevy Chase Club, that boring old institution, they were talking about bringing minority people in. They were talking about which ones. All the nonsense of my childhood has disappeared."

But there's one thing he doesn't quite understand. Alex Haley comes from "a distinguished Irish family" through his slave-owning great-grandfather. Debrett's discovered through research on the Jackson family - "why doesn't he look into that? All his ancestors are his ancestors, after all."