American Telephone & Telegraph Co. undertook an investigation earlier this year into the origins of a book highly critical of the giant utility.

Spokesmen for AT&T last night confirmed that a short-lived probe was begun and then dropped after the company purchased the book and determined that it lacked "proprietary information" promised in an advertisement.

In another development, informed sources said the Federal Communications Commission is looking into an attempt by AT&T's Washington subsidiary to bar former agency lawyers from participating in legal cases involving the nation's largest corporation.

The book at issue is "The Future of AT&T," published last year by Victor Schnee and Walter J. Gorkiewicz, who are selling it for $750 a copy.

According to AT&T spokesmen, when an advertisement for the book was published in The Wall Street Journal, a security officer at Bell System headquarters in New York noticed a claim that the publication would contain proprietary or private company information about AT&T.

The company's security officer, identified as Ray Ryan, subsequently asked representatives of New Jersey Bell Telephone Co., in Newark, to look into the matter. Ryan since has taken a new job with AT&T.

Pete Hoffman, general news manager of New Jersey Bell (an AT&T subsidiary), said last night that his firm received a note from Ryan on Feb. 8, asking: "What did we know about it" New Jersey Bell was contacted because the firm publishing the book, Probe Research Inc., had a Millburn, N.J., address.

On March 1, Hoffman continued, New Jersey Bell security department investigator James Arnold started to pursue the case and made a telephone call to the home of Probe Research circulation manager Rita Tannenbaum.

Hoffman said Arnold reached Tannenbaum's husband and began asking questions about the book and asked if he could visit the Tannenbaum residence, which also had a Probe Research business telephone.

Tannenbaum declined to be interviewed personally. Later, according to Hoffman, Rita Tannenbaum called the New Jersey Bell investigator and discussed the book.

Arnold told New Jersey Bell that the Tannenbaums claimed there was no proprietary information in the book and that many of the details previously has been published in The Washington Post, The Washington Star and Business Week.

After that, Hoffman said last night, Arnold did no more work on the case because AT&T notified its subsidiaries that it had purchased a copy of the book and could respond to any inquiries.

The Tannenbaums could not be contacted yesterday but the book's co-author, Schnee, said in a telephone interview from his home in Summit, N.J., that the March 1 call to his associate was the first of a "disturbing sequence of events."

He said another Probe worker has received "mysterious" telephone calls and the tires on his own car were deeply cut late last month. Summit police chief Thomas Finneran said Schnee reported the incident and that the police investigated but were at a loss to explain the slashings - which he described as more extensive than the normal punctures by vandals.

The AT&T and New Jersey Bell spokesmen said that so far as they knew, there were no further probes of Schnee or his associates after March 1, although no formal notice about discontinuing the investigation was sent, Hoffman said.

"I'm very worried about it," Schnee said.

In an unrelated case in Washington, involving AT&T subsidiary Chesapeake & Potomac Telephone Co., the D.C. Bar Association's ethics committee currently is studying a request that former FCC lawyers be barred from taking part in cases involving the Bell System.

C&P vice president and general counsel Robert A. Levetown filed ethics charges against Ruth S. Baker and Herschel F. Clesner, in the past 14 months, stating that they were in violation of bar association rules which forbid former government lawyers from participating in cases they worked on while in government employ.

Both Baker and Clesner worked on an FCC study of AT&T. Currently, Baker is with the law firm of Cohn & Marks, which specializes in communications cases and represents independent telephone equipment manufacturers. Clesner works for a House subcommittee, but represented citizens groups for free in a C&P rate cases in Maryland.

Baker declined to comment on C&P's action, as did her layer and officials of C&P. Clesner did not respond to a telephone call yesterday.

Informed sources said, however, that the C&P petition now is being studied by the D.C. Bar Association and that a decision may be forthcoming in about a week.

FCC staff members reportedly are looking into the C&P case as well, to determine AT&T's role - if any.

In a formal reply to the D.C. Public Service Commission, after a C&P complaint about her involvement in a case there, Baker said the local telephone firm seems to claim that "mere employment" as an FCC lawyer on a Bell System case "must automatically disqualify her from future employment in any matter, before any forum," that might include Bell System information, "whether or not these documents have the slightest relevancy to the matters at issue."