Simpson claimed they hooked up once after separating -- saying "the whole situation was very dark."
The memoir, out today, is filled with tid-bits from every stage of her life, including her volatile relationship with the 98 Degrees singer she was married to from 2002–2006.
In the book, she made it seem like they clicked immediately, with Lachey telling her he respected her decision to remain a virgin until marriage.
"He stayed patient, and how the relationship progressed physically was always up to me," she wrote. "He was the first guy who ever touched my breasts, and it was such a big deal to me that I made my mom take me bra shopping for the occasion. I spent an hour in a Victoria's Secret before I settled on a purple one."
The two would separate, briefly, but reconnected after the 9/11 terrorist attacks. As their relationship progressed, Lachey popped the question and she said yes. Before tying the knot, Nick suggested they get a prenup -- but Simpson said she "exploded" on him about the idea, so they didn't get one. Yes, we'll circle back to that later.
It was hardly wedded bliss for the two, as Simpson said they quickly found themselves in a "reliable cycle" of him criticizing her "for something small" and her blowing up to "make it about something later in our relationship or the pressure I was under in my career."
When "Newlyweds," the couple's MTV reality show, began, the public attention on their relationship skyrocketed. It also solidified Simpson's ditzy reputation, thanks to gaffes like the infamous "Chicken of the Sea" moment.
"Nowadays, we all know how much production goes into 'reality,' but back then, people believed," she wrote. "Of course, Nick had nothing to do with any of that, but I wanted people to believe he was everyone's vision of a leading man. The problem is, Nick could hit his marks, but he couldn't improv any lines."
As the relationship began spiraling downhill, Simpson became jealous and would "accuse [Nick] of having a wandering eye." According to her, that would lead to him saying she "was the one causing the problems in our marriage," before simply falling off the grid.
"He would go out of town and not answer his phone. Vegas or Miami with his boys. Or he would just stay out late to teach me a lesson," she wrote. "He had a group of guy friends who used him to get into places and enjoy VIP treatment at strip clubs and bars. He liked that scene, and I thought it was gross."
She recalled one time a producer on the show showed her a tabloid report about him doing "some vague thing with a porn star" simply to get a reaction on camera. As more reports like that one popped up, Simpson said she "just didn't know what to believe."
The two tried marriage counseling, with Simpson claiming Lachey simply stopped showing up. She also said he could be mean when he drank, saying he once told her, "Your friends don't exist. You just pay them to be around you. And your parents are only around because they are on the payroll."
When Simpson said she wanted a divorce, "he immediately tried to talk me out of leaving him," said Jessica -- who moved out when he was out of town, without telling him. "I wish we were the kind of people who could divorce and stay friends," she wrote, "We weren't, and I regret that my actions hurt him."
Following their split, Nick dove into work on his "What's Left of Me" album, which included a publicity blitz revolving around their divorce. "I watched him portray himself as a victim, casting me as this selfish person," said Simpson. "He then mentioned that the door was still open for us to get back together. It was so disrespectful and dragged me back into his orbit when I was just starting to leave it."
Simpson said that she eventually called him asking to meet, which they did at her home. He played her his album in full. "I didn't know any other way to make it better, so I slept with him," she claimed. "I know. I wish you were there to stop me, too. It was emotional, yet there was no connection. The whole situation was very dark. I didn't want the energy in my home. When he walked out the door, I knew I would never see him again."
While she didn't see him again, Simpson said Nick and her father -- who was still her manager at the time -- "continued to fight over how much money I had to give him" in the divorce. "They said Nick wanted a certain number, and honestly I don't remember what it was," she said, adding that eventually she told them to "just give it to him."
"I'll make it back," she recalled saying. "And then I did. Give or take a billion."
Lachey is married to Vanessa Minnillo, with whom he shares three children. Simpson later married Eric Johnson and has three kids with him as well.
In an awkward interview with TODAY this week, Lachey said he hasn't read the book and doesn't know what she wrote in it, but said he's "certainly happy for her and her life, I know she is happy for us."