According to the book, "Monica's Story," Lewinsky became involved with a man she identified only as "Thomas" after she was transferred from the White House to the Pentagon in April 1996.
At that time, her affair with President Clinton was on hold, and she didn't see the president again until early 1997.
Thacher Longstreth said yesterday that "Thomas" was, indeed, his nephew, Thomas Longstreth, 42, a handsome bachelor who has held a series of increasingly important jobs at the Defense Department.
He currently is deputy undersecretary responsible for assessing military readiness and training.
From 1995-97, Longstreth was director of the Defense Department's Bosnia Task Force, and it was in that job that he met Lewinsky.
Pushed out of the White House by aides fearful of her constant attempts to be near Clinton, Lewinsky was moved to the Pentagon and became part of a group that took a six-day trip to Bosnia and other European stops.
In the book, Lewinsky said her contacts with Longstreth on that trip led to them dating, and the relationship quickly became physical.
It lasted only three months, Lewinsky said, but as the affair was ending, she discovered she was pregnant.
According to the book, Lewinsky and Longstreth "fell out" over the costs of an abortion she decided to have, and she borrowed the money from her aunt, Debra Finerman, a Virginia writer.
A source with knowledge of the matter contradicted that assertion, describing Longstreth as "a gentleman" who paid for the procedure and has documentation to prove it.
"He is a brilliant man who is having a brilliant career, and he's also an incredibly charming person," this source said.
"He knows what the right thing to do is and he does it."
Thacher Longstreth said he was distressed about the public airing of the abortion, describing it as "ugly."
He said his nephew and Lewinsky should have been able to have a relationship and end it without public notice.
The book, written for Lewinsky by British author Andrew Morton, said the former White House intern wanted an abortion because "she was convinced that she did not want to be a single parent."
Lewinsky said she told Clinton she was seeing another man during the period she was barred from the Oval Office.
The book is silent on whether she told Longstreth about her relationship with the president.
Longstreth, who has refused to talk to reporters about Lewinsky, is a 1979 graduate of Wesleyan University in Connecticut with a degree in American History. He has a master's degree from George Washingto University's School of Public and International Affairs.
Longstreth joined the Pentagon in 1990, where he worked as a planner on the Persian Gulf War.
Thacher Longstreth recalled attending an event at which the principal speaker was former Joint Chiefs of Staff Chairman Colin Powell.
"When he heard my name, he came down off the platform and shook my hand," Thacher Longstreth said. "He told me what a tremendously talented young man my nephew was."
Thomas Longstreth's parents couldn't be reached.
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