Houghton Mifflin Co.'s publicity and advertising director Carolyn Amussen was watching three of the TWA hostages giving a television interview on June 27 before they were released and heard the unofficial hostage spokesman, Allyn Conwell, say in reference to the Middle East problems, "Learn and take part in it." And in that small phrase she saw an advertising campaign.

Conwell's quote in large black type was on an ad for Jimmy Carter's Houghton Mifflin book "The Blood of Abraham" that appeared yesterday in The Washington Post, The New York Times, the Chicago Tribune and The Houston Chronicle. The text touts the value of the former president's book in understanding the insights of the Middle East, saying, "As Allyn Conwell so eloquently stated, peace in the Middle East will depend upon an understanding of the issues and enmities that divide it."

Erin McHugh, an advertising executive at Franklin Spier Inc. in New York, said Houghton Mifflin's lawyers said the quote was usable without consulting Conwell, who lives in Houston. Nor was Carter consulted, since he does not have advertising approval in his contract. And, Amussen said, Houghton Mifflin was careful not to use Conwell's quote as an endorsement.