By Emily Strohm
April 01, 2016 at 12:00 PM EDT
Dimitrios Kambouris/Getty Images

This article originally appeared on

When Anderson Cooper and his mom Gloria Vanderbilt began emailing back and forth last year, neither of them imagined that their conversation would turn into a full-blown memoir with secrets of their legendary family filling nearly every page.

“I think everybody as you get older and your parents get older, the relationship changes, but often times you don’t come up with a way of talking or a new way of discussing things,” Cooper explains in a recent interview with PEOPLE and Entertainment Weekly editorial director Jess Cagle.

“When my mom turned 91, I wanted to use the time that we have left in our lives to kind of get to know each other as adults.”

What transpired was a deeply personal exchange that led the mother and son to learn things they never knew about each other and become closer than they ever dreamed.

“What’s interesting is I think that there is something about email that frees you up from any embarrassment or awkwardness or you’re able to talk about things you wouldn’t ordinarily talk about,” says Coooper.

While Cooper and his mother have few secrets left between them, Vanderbilt still manages to surprise her son.

“When I was 13,” Vanderbilt reveals in her PEOPLE interview, “I had a brief lesbian relationship with a girl in school. It felt so great.”

“What? Hello! This is news to me,” Cooper says with a smile. “You didn’t mention this in the book, Mom.”

Cooper and Vanderbilt’s memoir, The Rainbow Comes and Goes: A Mother and Son Talk About Life, Love, and Loss will be available on April 5, and an HBO documentary airs April 9.