O'Regan shares fond memories of Bowie with PEOPLE ahead of the release of David Bowie on Tuesday, August 5.
Denis O'Regan had an unlikely road to his place as one of rock's most iconic photographers.
In his new book David Bowie by Denis O'Regan, the famed photographer takes a heartfelt and meticulous look back at touring with the iconic singer over three decades -- in the 1970s, at the end of his Isolar II World Tour; in the 1980s, on the Serious Moonlight and Glass Spider world tours; and in the 1990s, during the Tin Man and Outside tours.
Speaking with PEOPLE about the collection of photos and stories that comprise the book, O'Regan opens up about what it was like getting to know Bowie.
"When I got there, I thought David would be seen in these different guises, and no one had ever really seen him offstage that much. I thought, 'Well, he's going to keep us [at] arm's length. He won't let me do this. He'll be demanding about that,'" he recalls. "And of course, that wasn't the case."
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O'Regan was shocked by the "Rebel Rebel" singer's comfortability giving him "unfettered access to everything, everywhere and anywhere."
"I just did what I did and captured it all, and we got along," he shares. "That really helped. I was fairly lazy, so that also helped because I wasn't constantly hassling him or anyone I worked with to take pictures."
Getting to know Bowie was an experience in itself. O'Regan found himself "surprised" by "how pleasant he was and unprepossessing."
"He was very normal and that's not what I expected from the person I'd seen on stage. He was still this English lad who hid a cigarette in his hand behind his back like he was a naughty schoolboy," he recalls.
"He was also very funny. He loved to imitate people and he laughed continually. And that I didn’t expect. So I didn’t expect him to be so friendly and accommodating really is what it was, and so enthusiastic about what I was there to do."
The photographer came to appreciate Bowie's character, on and off stage.
"From David being around all the time, he had gone from this person who was an enigma for me and this hero, to there I was, with him all day, every day. Sometimes I did wonder how I'd managed it."
O'Regan also got used to seeing other stars, from those who worked with Bowie to those who were simply fans of the musician and would come to his shows. On one occasion, the photographer was preparing to shoot a show at Wembley Stadium when he learned that Princess Diana was coming and eager to meet Bowie.
"I thought, 'Well, Princess Diana, that's fun.' But there was no communication. I was out in the audience at Wembley Stadium, and I took my father and my brother out there to get them a good place, and we were gone for quite a while," he recalls. "Then I came back and David and his PA just said, 'Diana’s on her way.’ "
O'Regan says Diana was with "a friend," who would later be revealed to be Army Major James Hewitt, though, at the time, no one had any suspicions the two were romantically involved.
"On that day, she was just with a friend. And it was only when that was reported over the next couple of days that we really knew what was going on," he adds. "It didn't make any difference to our day, but that's what happened."
When Diana arrived, O'Regan asked promoter Harvey Goldsmith if he could photograph her. Goldsmith then told him to "ask her yourself."
The photographer remembered feeling like "protocol went out the window."
"I thought, 'Okay, wasn't quite sure that's how it's done.' So I did and I said, 'Would you like a picture taken with David?'" he recalls.
"And she said, 'Do you think you'd really want one taken with me?' And I went, 'I think he would actually,' so then we did it. But it was lovely, and she was lovely, and it was great to have done it."
Another unforgettable celebrity sighting was when Michael Jackson spent time backstage with Bowie -- narrowly missing Prince, who "scuttled off just before" his fellow pop star arrived.
"With Michael Jackson, everyone was just milling around in the green room, and Michael didn’t want any pictures taken," he recalls. “So I said, ‘Well, it’s either pictures with David or it isn’t,’ so he agreed, and that was the first time I met.”
“I think the thing that surprised me about Michael Jackson was he was quite tall, and that threw me,” O’Regan shares. “He was coming across as this little boy. If you look at the piece, he’s as tall or taller than David.”
It wasn't unusual to see the stars showing up to greet Bowie at any gig. Noting there were "always people milling about," O'Regan saw famous faces including Andy Warhol, Duran Duran, and Gary Oldman.
"I'd wonder if they were fans or if they just wanted to meet him or get their picture taken with him," he admits.
"Mick Jagger was an old friend, so I photographed them together more than once during those tours. And one picture of David and Mick; it’s in the book; but I took it at a club after a Wembley Stadium show; and it’s Mick and David sitting at the table. And when David passed away; Mick tweeted that picture as his tribute."
Similarly, Madonna used a photo taken by O'Regan to commemorate Bowie's death in January 2016.
"She just cropped herself and David out; but to the left; there was Sam Kinison; the comedian; and next to him was Billy Idol. I know David was a hero to Madonna because Sean Penn told David that Madonna based her entire career on David; and the change of look and things like that; so that must’ve been quite a moment for her," O'Regan says.
The photographer found that, like himself, "loads of people from all different walks of life" adored Bowie.
David Bowie by Denis O'Regan is available wherever books are sold beginning on Tuesday, August 5.