Why Laura Dern Is Still 'So Angry' After Mom Diane Ladd Took Son to Get His Long Hair Cut (Exclusive)

Laura Dern and Diane Ladd's new joint book, Honey, Baby, Mine, is a collection of intimate conversations between the two when they thought Ladd only had six months to live.

Diagnosed four years ago with a lung disease called idiopathic pulmonary fibrosis, Ladd, now 87, began going on walks around Santa Monica with her daughter to try and strengthen her breathing. Their discussions as they walked — including everything from their own journeys as actors to grief and heartbreak in their lives —"grew into a profound deepening of our relationship," Dern tells PEOPLE in this week's issue.

It also opened a can of worms.

Chatting with PEOPLE to promote their book, the two were asked what conversation made them laugh the hardest. "I think it was discussing one of our worst fights. There's a chapter called The Haircut," Ladd says. "We still don't agree, by the way." Her daughter concurs: "We are still both so angry, and we are still both absolutely confident that we are right."

In the book, Dern, 56, brings up a memory of her son Ellery, then 6 years old, being taken to get a haircut by Ladd while Dern and her ex-husband Ben Harper were out of town. According to Ladd, Ellery — who had long hair — asked for the cut. Dern insists Ladd and her husband Robert Charles Hunter convinced their grandson to do so.

"You made Ellery embarrassed," Dern wrote to Ladd. "Thinking that boys can't have long hair is just so outdated."

Ladd, who had taken her grandson to get his hair trimmed before, was taken aback by her daughter's response. "God almighty, you and Ben went crazy, like I'd gotten him a tattoo," she wrote in the book.

Afterward, she left Dern's house "in a huff" parked the car, and sobbed. "You treated Grandmother wrong," she wrote. "I would have never treated my mother that way if she'd cut your hair. And she did cut your hair! Many times! I didn't always love the haircuts, but so what? I loved her."

Dern recalls being "devastated. It wasn't just the haircut; it was that you said he looked like a girl," she wrote. Ladd insists she never said that to her grandson, only to Dern.

In the book, Ladd allows that the incident still bothers them both. "Did Ellery want it cut? Or did he just know I wanted him to cut it?" she wrote. "All you can do is hash it out and know where you stand and agree to disagree."

After Ladd revealed to PEOPLE that one of her proudest moments of Dern was when she gave birth to her kids, Dern had an addendum: "That's so beautiful, Mom. I just want to add, if I may, because I was so moved, that you said when your child gives you a child. That doesn't mean you get to cut its hair!"

"A mother's wisdom understands that that's what the child wants," Ladd responded. "Here we go."

"In every interview, I get to reiterate how wrong you were about cutting my son's hair!" added Dern, laughing. "This book was so worth it."

Honey, Baby, Mine is available now.