"It will include my experiences, everything that's taboo to talk about," Remini, 43, told Us Weekly magazine at a charity event July 27.
Remini, former co-host of "The Talk" and star of the sitcom "The Kings of Queen," joined the church at age 7 and served for one year in its elite "Sea Org" religious order. She was also friends with some of Scientology's most high-profile members, including Tom Cruise.
"I think this would be one of the most sought-after celebrity memoirs of the last few years," said Andy Lewis, book editor for The Hollywood Reporter. "I think every publisher will be interested in bidding for it."
As a celebrity breaking Scientology, Remini follows in the footsteps of "Crash" director Paul Haggis, who left the church in 2009. Remini's split is seen as a bold move, something confirmed by Haggis himself.
"The stakes for her were so much higher than for me," Haggis wrote in an open letter published last Thursday. "Her decision to leave was so much braver."
In a statement, the Church of Scientology, which had declined to comment to ABC News about Remini's departure, called Haggis a "status-obsessed screenwriter" who is "exploiting his tenuous connection with Scientology to grab headlines."
A memoir from Remini would be the actress's first insight into the reason behind her departure. In a statement at the time, Remini said only that she was, "truly grateful and thankful" for the support she had received from, "the media, my colleagues and fans from around the world."
Her older sister, Nicole Remini, says that her sister was rebuked by church officials after asking the head of the church, David Miscavige, who was Cruise's best man, where his wife, Shelly, was. Shelly Miscavige has not been seen in public for years.
"According to Leah's sister, when Leah asked if she could call Shelly Miscavige, she was asked to write a letter and that the church would hand it to her," People magazine's Raha Lewis told ABC News July 18. "The church claims that Shelly Miscavige is a private person and just does not like to be seen in public."
The church has publicly denied the story that Remini confronted David Miscavige at the wedding.
Remini has not confirmed her sister's account to ABC News but the outspoken star did tell People magazine that she won't be kept quiet.
"I believe that people should be able to question things. I believe that people should value family, and value friendships, and hold those things sacrosanct," the actress told People magazine at a charity event last month. "That for me, that's what I'm about."
"It wouldn't matter what it was, simply because no one is going to tell me how I need to think, no one is going to tell me who I can, and cannot, talk to," Remini said.