THE TITLE of a little book published earlier this month -- "Birth of a Salesman: Lawyer Advertising and Solicitation" -- would have been enough, just a decade ago, to have driven the nation's legal establishment mad. What the book contains would have been regarded as rank heresy in those bastions of legal purity along Wall Street and 15th Street. One chapter, for example, is full of advice to lawyers on how to write better ads. Other chapters conclude that Advertising is good for lawyers, good for their clients and good for the law. Yet the publisher of this little book is not some wild-eyed, left-wing consumer group but the American Bar Association.

That alone suggests how far the legal profession has come in the 3 1/2 years since the Supreme Court sanctioned some kinds of advertising by lawyers. It is not suprising. Legal advertising has in fact flourished, and lawyers (naturally) have learned how to make money out of it.

The key findings by author Lori B. Andrews, however, are of broader interest. They demonstrate that since lawyers began to advertise, legal fees for the services used by most people have gone down. Wills, divorces and real estate settlements cost less now, in many communities, than they did five years ago.

How, them, are lawyers making more money? Advertising and lower prices have stimulated, just as the textbooks say they will, new business. More people have discovered that they have legal rights they can afford to exercise. This has not resulted, Miss Andrews reports, in unwarranted court cases. Instead, it has brought complaints from other professionals, among them the bankers and real estate agents, that consumers are exercising their legal rights too often and too vigorously.

Twenty years ago, the legal rights of only the well-to-do were protected in both criminal and civil matters. Since the advent of government-funded legal services for the poor, high-quality protection has begun to be available to those at the bottom of the economic heap. That has left the great middle class with a lot of rights but neither the knowledge nor the money to use them. Advertising -- not just in newspapers but on television and through other methods -- seems to be filling that gap without, according to Miss Andrewss, causing any drop in the quality of the legal services rendered.

Lawyers have always preferred to be regarded as professionals. If advertising is forcing them to become salesmen, so be it. We look forward to the day when they start peddling their wares door-to-door. Given the way they are rolling out of law schools these days, that time may not be far off.