Former Iran-contra prosecutor Jeffrey Toobin and his publisher protested yesterday that his former boss, Independent Counsel Lawrence E. Walsh, was improperly stretching the grand jury secrecy rule in an effort to prevent Toobin from expressing his opinions.

In a brief filed in federal court in New York, Toobin, who has written a book about the Walsh investigation, assailed Walsh's objections to the manuscript as "preposterous" and "inconsistent with any good-faith concern about the disclosure of grand jury material."

Walsh submitted his complaints about specific passages in the book, covering about a third of it, in a sealed filing last week with U.S. District Judge John Keenan.

Toobin and his publisher, Penguin Books USA Inc., then submitted a sealed reply and followed up yesterday with an edited version that was made public. The book, "Opening Arguments: A Young Lawyer's First Case -- United States v. Oliver North," has been scheduled for bookstore sales in February, but Toobin and Penguin went to court last month to seek protection from what they said was a threat of criminal prosecution and civil penalties.

Walsh said earlier this week that he was not seeking "prior restraint of publication" but argued that the 30-year-old author and the publishing house should be subject to legal sanctions if they bring the book out. He maintained that the book, billed as "an insider's blow-by-blow account," contains materials protected against disclosure by the grand jury secrecy rule and by Toobin's obligations as a member of the New York bar and a former employee of Walsh's office.

Lawyers for Toobin and Penguin said in their brief that Walsh's office had marked about about 114 pages, about a third of the book, as out of bounds, including about 57 pages under the grand jury secrecy rule.

The rule, known in the courts as "6(e)," applies to "matters occurring before the grand jury."

The Penguin-Toobin lawyers called Walsh's claims on this score "astonishing." Most of the independent counsel's objections, they said, deal with factual material available from public sources "and not linked in any way in the book's text to a grand jury" or with expressions of opinion and discussions among members of the Walsh staff.

"Indeed, {Walsh and his office} repeatedly contend that expressions by the author even of his own opinions . . . are 'grand jury materials' protected from disclosure by Rule 6(e)," the brief said.

While Walsh has asserted a broad coverage for the rule, the Penguin-Toobin lawyers said previous court cases show it does not extend to facts previously made public, documents obtained by means other than a grand jury subpoena and witness interviews that did not take place pursuant to a grand jury subpoena or in the presence of a grand jury.

The brief said there were only three segments of the book "which even arguably reveal what actually took place in a grand jury" and "each of them has been publicly disclosed in a judicial proceeding or opinion."

Toobin and Penguin also maintained that a non-disclosure agreement Toobin signed in May 1989 on his last day of employment with Walsh was unenforceable and that Walsh had in any case made it invalid by failing to specify his objections to the manuscript "promptly upon receiving it."

Prompt action is necessary because such agreements "are prior restraints on speech," the brief said. "Here it was more than 200 days after receipt of a complete draft of 'Opening Arguments' that {Walsh's office} first specified their objections in confidential settlement discussions."