Charles Ramsey signed the deal with the Cleveland publisher David Gray & Co. on Thursday.
Amanda Berry, Gina DeJesus and Michelle Knight and Berry's 6-year-old daughter escaped to freedom May 6.
Collaborating as Ramsey's co-author will be freelance writer Randy Nyerges.
The former U.S. Senate staff speechwriter co-wrote "Day of the Dawg," with former Cleveland Browns defensive back Hanford Dixon.
"What you saw on TV doesn't even begin to tell the story," Ramsey said in the company's announcement of the signing.
Ramsey and Nyerges started work on the book early this month, David Gray said. Ramsey, who had been working as a dishwasher, is devoting full time to the project.
"Charles says outrageous things, but what a story he has," Nyerges said. "America doesn't know yet how truly brilliant this guy is."
The book, which does not yet have a title, will be published next spring.
"He's completely unfiltered," Gray said, according to The (Cleveland) Plain Dealer (bit.ly/1dvRJZt). "I think that's part of his appeal. He says what he thinks. I am really intrigued by him -- as a person, with the story he had to tell, and with his ability to tell it."
Last May Ramsey, 44, heard screaming from Ariel Castro's Seymour Avenue house next-door. Ramsey ran over, helped a woman who said she was Amanda Berry escape through the front door, and called 911.
He was hailed as a hero, and his animated TV interviews, offering blunt opinions on race, class and life in the inner city, made him a sensation.
Seeing a white girl in that situation was "a dead giveaway" that she was either homeless or had other problems, he said.
"When a little pretty white girl ran into a black man's arms, something was wrong," he said.
Viking announced last week that it has acquired a planned book by Berry and DeJesus. The book is currently untitled and is scheduled to come out in 2015.
Knight is working on her own memoir, which Weinstein Books plans to publish next spring.
Castro was arrested and eventually sentenced to life in prison. He was found hanged in his cell in September.
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