I found this interview that David Bailey gave, it's great, here is the portion that focuses mainly on Penelope...
BAILEY: I can tell you this: size doesn't count. It got better, too. There were a few in between, by the way. I remember Catherine saw some pictures of Penelope Tree by Avedon in American Vogue, and she said, "You're gonna run off with this woman." Which was odd because I didn't even know who Penelope Tree was.
But you did run off with Tree. And wasn't she 17 years old?
BAILEY: Well, I was only 30—or something like that. Yes, Penelope was 17, but mentally she was 35. I mean, Penelope was very grown up—really sophisticated.
Was she a similar kind of phenomenon as Twiggy?
BAILEY: Penelope was more than Twiggy. Twiggy was like the Monkees, the Beatles. Penelope kind of started all that "Flower Power." And she wore the shortest miniskirts I've ever seen.
Wasn't she aristocratic?
BAILEY: Yes, and she was a real rebel. But I didn't do such great pictures of Penelope. Somehow I couldn't. Avedon did great pictures of Penelope—really great pictures. But I guess Penelope's still my best friend, along with my wife and a couple of guys. I see her at least once a month.
How did you meet her?
BAILEY: Vogue called me up and said, "We're photographing this very aristocratic girl called Penelope Tree, and we don't want any of your nonsense." What a stupid thing to say. It was like a red rag to a bull. If they hadn't said anything, I might not have noticed. But because they said it I thought, "My God, now I'm really interested."
Was she as bright as they say?
BAILEY: Bright, bright, bright, bright. I think the first conversation we had was about T.S. Eliot. And it didn't stop for eight years.
What was it about her look that made her right for that time? Why did she hit the way she did?
BAILEY: In a way, Penelope was New York's revenge on London. It was sort of, "We can shock too." Vreeland discovered her at Truman Capote's Black-and-White ball. Actually, I think Guy Bourdin was the first one to take pictures of Penelope, then Avedon. And it went from there.
How happy were her parents to see you coming?
BAILEY: Her mother hated me. But her mother was a horrible woman. Marietta Tree, my God, what a bitch! She was a complete phony, a fake, a snob,... the worst!
Do you think that made you more attractive to Penelope?
BAILEY: I think she liked it that I didn't like her mother, in a funny sort of way, because I remember when I went to collect Penelope, her mother opened the door and said, "She's not going to London with you." And I said, "Oh, all right." Then I said, "You know, it could be worse; it could be a Rolling Stone." And she laughed. So we sort of had a standoff. Anyway, Penelope and I flew off to London, leaving tear-stained Marietta Tree on the doorstep of her mansion.
Didn't she give up Sarah Lawrence for you?
BAILEY: I think Penelope would have given up Sarah Lawrence anyway. In a way, she was sort of an intellectual cripple. She wanted to write like Dostoevsky straight away. She was too self-critical... she wanted to start there and not work up to there. Which paralyzed her. I think some people get paralyzed because they're too bright. It's best to be a bit stupid like me and not know that you're not any good. Nobody's any good at the beginning. But unlike Shrimpton and Deneuve, Penelope was fantastic in terms of style—avant-garde. When she was 19, they offered her the editorship of Italian Vogue. Penelope was out there! You should have seen her father's face when she walked out with a skirt made of raccoon tails, and her knickers below her skirt! But Penelope's father was great. He once said, "We're almost the same because we're so far apart that the circle's almost complete." But Penelope was something special. I think she changed a generation of young American girls.