The 40-year-old singer and fashion designer said on Tuesday's episode of Good Morning America that Johnson, 41, "read every chapter" and cried with her as she wrote her intimate memoir, Open Book.
Simpson and Johnson, a former professional football player, married in July 2014. The couple have three children, son Ace, 7, and daughters Maxwell, 8, and Birdie, 2.
Simpson released a new, paperback edition of Open Book on Tuesday. The book explores Simpson's personal and professional life, including her past sexual abuse and her struggle with alcohol and drugs.
On GMA, Simpson said Johnson supported her as she revisited journal entries and attended therapy while writing the memoir.
"Through the process of getting sober and stopping all the drinking, I really wanted to go back to my childhood. I wanted to go back to who I was and rediscover that person, and who I am now," Simpson said.
"He really just nurtured my character along the way," she said of Johnson. "After every therapy session I would have and the journal entries I would go through, I would share with him. It just ended up becoming a memoir."
Simpson said she wrote the book so the world "can understand me in a different way and on a different level and on a deeper level."
"Eric, he's a very open person like I am. He supported it. He read every chapter with me, he cried with me, he laughed when I didn't," she said. "He's very supportive and he's very proud."
Simpson said she "can't wait" to experience the new opportunities in her career with Johnson and their family.
The new edition of Open Book includes diary entries that Simpson wrote following her split from her ex-husband, singer Nick Lachey. Lachey started dating his now-wife, Vanessa Lachey, shortly after the split.
"So, Nick, you're with another already? Seems that you forgot the love you spoke to me," Simpson wrote in one entry. "I'm saddened beyond belief. Alone in the dark, with no one to call my own."
The new edition also addresses Simpson's mom jeans controversy and features a new introduction, in which Simpson writes more about the sexual abuse she experienced as a child.