NEW YORK, AUG. 16 -- Fashion mogul Oleg Cassini says he had to set up a separate operation devoted exclusively to Jacqueline Kennedy Onassis because as first lady she insisted on originality, according to People magazine.

The magazine, in what it said was an exclusive excerpt of Cassini's autobiography, "In My Own Fashion," quoted the designer as writing that, "designing for Jackie posed an invigorating challenge. Her taste was so demanding."

Cassini's book says Onassis, who married Greek shipping tycoon Aristotle Onassis following the death of her husband, President John F. Kennedy, had cherished original clothing. She told Cassini she did not want any "fat little women hopping around in the same dress."

"I really don't care what happens later as long as when I wear it first it is new and the only one in the room," she told Cassini.

People quoted the book as saying Onassis wrote a letter to Cassini with instructions on how to begin designing her clothes and where to have accessories made.

She warned Cassini, who designed more than 300 outfits for her during the Kennedy administration, to protect his designs from others who might try to produce cheap imitations.

Cassini said of Onassis, now 58, "This quiet, private force of personality was quite in contrast with the rather shy public presence, the tiny, whispery public voice.

"But then, Jackie was famous for her elusiveness, her shifting moods. She might be very warm one day, and freeze you out the next ... You never really knew where you stood with Jackie."