A dark family secret was kept from Amy Duggar King for many years.

While speaking to PEOPLE about her debut memoir Holy Disruptor, out Oct. 14, the 19 Kids and Counting alum opens up about an unexpected family revelation that she learned after the death of her grandfather, Jimmy Lee Duggar.

Amy, now 39, was not allowed to be alone with her grandfather at any point when she was growing up in Arkansas. “I wondered all the time. I asked. I asked my mom, I asked my grandma, and they obviously were going to protect the answer. They didn’t want me to know that information growing up,” she says.

After his death, Amy’s mom sat her down to reveal the truth, that her late relative was a “predator,” as Amy labeled it in her new book.

“I honestly did not know why until my mom and I spoke after he passed away,” she shares. “But I obviously assumed. I always had assumptions as to why, but it was never spoken about. It was a little difficult, but I’m one of those people that when I was little, I asked questions, sure. But if something was told to me, I just believed it.”

“I was very naive, and I’m a kid, so I was like, ‘Okay,'” she continues. “I knew grandpa couldn’t go to the trampoline with me. I knew grandpa couldn’t sit and watch a movie with me. I knew grandpa couldn’t be in a car with me. I knew grandpa couldn’t take me to school. There were so many things, and I just knew that that’s how it was.”

As a protective measure, her now-deceased grandmother would ensure Amy’s childhood bedroom door was locked each night. “Every single night,” Amy says. “My mom would check on me as well. It was like this protective tent over me, if you will, that was not going to let a bug in.”

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In knowing the truth now, Amy says that “it changes so much.”

“My mom and I sat there and cried, and I just held her, and she held me,” she recalls. “It was like this insane bonding moment that we’ve been needing for our entire life, that we finally were allowed to just be really vulnerable with each other and share. And thank God.”

“I mean, looking back, oh my gosh, I could have been a victim too. It’s crazy to think it was so close to me in so many ways.”

Other revelations in Amy’s book include stories about her cousin Josh Duggar, who is currently serving a nearly 13-year sentence for receiving and possessing child pornography, charges of which he has denied and is attempting to overturn.

Josh, now 37, previously admitted to molesting multiple females in the early 2000s — four of whom were his then-underage sisters.

Although Amy didn’t get to confront her grandfather about his alleged misconduct, she did speak to Josh about his wrongdoing against his sisters.

Amy notes that after she asked Josh why he didn’t hurt her, she said he “knew better” than to do so. Afterward, she says Josh expressed “the creepiest smile.”

“In that moment, I just looked at him, and I was just like, ‘I don’t know you at all.'”

Sharing these experiences and more in her book was no easy feat for Amy.

“Writing was the most difficult thing I’ve ever had to do,” she shares. “I pushed myself to my limits. I cried all the time writing it, but I just wanted to be brave and tell the truth.”

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Holy Disruptor: Shattering the Shiny Facade While Getting Louder with the Truth is out on Tuesday, Oct. 14 and available for preorder now, wherever books are sold.

If you or someone you know has been a victim of sexual abuse, text “STRENGTH” to the Crisis Text Line at 741-741 to be connected to a certified crisis counselor.