There was a time when getting information about the inner workings of major law firms was as difficult as learning about what goes on in judges' chambers. Reporters invariably were met with incredulous stares from members of the tight-lipped, three-piece-suit fraternity. Theirs was not the public's business.

But the times are changing. A new book on law firms published by The American Lawyer, a monthly trade magazine, confirms the new era. The book is a quantum leap in reporting on the most influential profession in the country.

The 970-page Guide to Law Firms, though not written in the breezy style of the monthly, provides detailed information on how 172 of the nation's largest and most influential firms in 13 cities are organized, who their clients are, how much they charge, how they are governed and how they have fared since formation.

For outsiders, it is excruciatingly dull stuff, on the order of the bland Martindale-Hubbell law directory, which lists essentially name, rank and serial number of the lawyers in firms across the country.

It is not an investigative report. It covers only the largest or most influential [in the publisher's view] firms in only 13 cities. The four-to-six pages devoted to each firm really don't say if a firm is worth what it charges.

The firms are, by definition, high-powered and successful. The guide makes no attempt to critique their relative skills or construct a won-loss record so shoppers can determine which firms are the most successful.

But for the law school student who wants to know how a prospective employer handles its young associates, for a lawyer who wants to get an idea about just who his opponents are, or for a corporation counsel looking for a firm to handle a particular case in one of the 13 cities listed, the guide is an important companion to the more sterile and far more limited information in Martindale-Hubbell.

Many firms, corporations and schools will find the profiles an important supplement to Martindale, perhaps even worth the $390 price tag.

Editor Steven Brill does not claim the guide is an investigative report. But given the lack of information on law firms heretofore available and the reluctance of firms to discuss internal affairs, the book is a prodigious reporting effort.

The American Lawyer, which had five reporters working full time for a year on the project, overcame the firms' nearly universal resistance by using the time-honored reporting technique known as journalistic extortion.

"We said, 'You really should cooperate with us because the book is going to be in law schools' placement offices, so help us make it accurate,' " explained Brill, who pointed out that several drafts were sent to the firms for review before publication. "Once they saw we were serious, they had to cooperate," Brill said.

"We despaired of the whole project," said Covington & Burling partner Edward Dunkelburger, "but the choice was to say we had nothing to do with it and let it be published with errors or at least correct what they had." The first draft, he said, had "everything wrong."

Brill said that by the end of the project, only one firm refused to cooperate. "There was one rule," Brill said. "No firm could get out of it because they didn't want to help us." In one instance, he said, he had decided not to review a New York firm only to find out that a reporter had approached the firm and had been rebuffed. So Brill said he decided to include that firm in the book.

Arnold & Porter managing partner G. Duane Vieth said his firm decided to correct obvious errors, but did not add anything to what the reporters had. Other firms likely did the same, which would tend to make the book essentially accurate, but not necessarily complete, at least in the section where key clients are listed.

"Most of us trained in law in the old days don't seek publicity and don't approve of it," Vieth said.

Despite all the harrumphing from the firms, the guide is in fact excellent publicity for them. Sometimes the profiles read like advertisements.

It can hardly hurt Covington to have someone read that it "appears before more federal agencies than any firm" in the city. Patton, Boggs & Blow is hardly in a position to sue for libel for being called "one of the premier lobbying firms in the city -- largely due to the prominence of star lobbyist Thomas Boggs Jr."

Arnold & Porter is reportedly miffed that the "other notes" section included in the profile said "the firm suffered a cash shortage in 1980 and 1981 because of expenses incurred in the construction and furnishing (including a spectacular art collection) of a plush new building . . . "

That note, Brill concedes, was not shown to the firm before publication. But the same paragraph calls A&P "one of Washington's most solid, well-balanced firms . . . with a large stock of smart, well-credentialed . . . associates."

Brill says most of the 750 copies sold so far have been to corporations' in-house legal departments, which will use the guide when looking for a law firm to represent their interests on a particular matter or in a particular area of the country. A profile in the guide is an obvious draw.

Brill says he's been hearing from lawyers who are fascinated to read how other firms govern themselves, information that law firms rarely if ever trade with each other. Although most firms are generally divided into partners and associates, some operate like New York's Stroock & Stroock & Lavan, which has 61 partners but, according to the guide, "18 senior partners who effectively dominate the firm."

Selling only 750 copies does not make the book a best-seller, but at $390 each, that is a lot of income. The first printing was 1,000 copies, with break-even for the project set at about 400 copies, Brill said. It has been "terrifically successful." A second printing may be ordered.

In addition, the magazine is already at work on the 1982-83 edition, planning to add six more cities and more law firms to the guide, which will be published as a two-volume set.

The revenues from the book should be more than enough to ensure the continued survival of the monthly publication, which has not done as well financially as Brill and publisher Jay Kriegel had hoped.

Dunkelberger calls it "an interesting browsing document" and predicts at least 172 buyers. "Any firm mentioned would be crazy not to buy it," he said.

"We might have liked the old days better," Dunkelberger said, "but the new days are here."

Fourteen Washington firms are listed: Arent, Fox, Kintner, Plotkin & Kahn; Arnold & Porter; Covington & Burling; Hogan & Hartson; Howrey & Simon; Leva, Hawes, Symington, Martin and Oppenheimer; Patton, Boggs & Blow; Shaw, Pittman, Potts & Trowbridge; Steptoe & Johnson; Surrey & Morse; Verner, Liipfert, Bernhard and McPherson; Wald, Harkrader & Ross; Williams & Connolly; and, Wilmer, Cutler & Pickering.

The guide lists information for each firm according to an identical format, starting with a profile giving data such as the size of the firm, the average age of the partners, starting salaries, associate turnover and billing rates.

So, for example, the guide says 25 percent of the lawyers at Patton, Boggs went to Harvard Law School and 31 percent of new associates come from there. The lowest hourly billing rate for associates is $60, the highest partner rate is $200 an hour. One third of Williams & Connolly's new associates went to Yale, although Harvard graduates dominate among the firm's 60 lawyers.

The firm-by-firm section on key clients and responsible partners for those clients lists, for example, 31 clients for Wald, Harkrader, including: Air Florida, with Judith Hope and Robert Lichtman working for that client; the American Iron and Steel Institute (partners Thomas Truitt and David Berz); the Embassy of Great Britain (Mark Joelson and Joseph Griffin); Ford Motor Co. (Robert Wald, Robert Skitol and Thomas Brunner); Occidental Petroleum (Wald and Donald Bucklin; and Newsweek (Wald, Toni Allen and Stephen Truitt).

Howrey & Simon, with 22 percent of its 120 lawyers coming from George Washington and Georgetown, has 26 key clients listed, including Exxon (William Simon); Ashland Oil (Ray Bolz); Black & Decker (David Murchison); Litton Industries (Simon and John Bodner Jr.); Mobil (Edward Howrey and A. Duncan Whitaker); Rockwell Industries and Shell Oil (J. Wallace Adair, James Fox and Roger Simmons); Texaco (Adair) Getty Oil and Gulf Oil (Adair and Simmons. Faberge and General Mills are handled by Ralph Savarese.

The more gossipy "Other Notes" section is sure to be widely read. In that section, under Verner, Liipfert, the guide says the firm "astounded Washington legal observers in 1979 by acquiring client Pan Am from Jones, Day, Reavis & Pogue," dropping smaller but longtime client Northwest Airlines to avoid a conflict.

The notes for Wilmer, Cutler say that the firm has an ombudsman, James Robertson, who is "responsible for taking complaints of associates to the partnership." One common complaint at Wilmer, Cutler and other major firms, the note says, is that newer associates aren't given much responsibility.

The notes for Arent, Fox include a remark that "local lawyers say the firm is well on its way to a second major expansion," with the "significant lateral entry" of Arnold Weiss, former general counsel of the Inter-American Development Bank.

Gossipy tidbits like those, solid information on what the firms do and how they do it, and a wealth of other material don't necessarily make the guide "must reading," but it is indeed a very good browse.