Cox said it's unclear whether the family will profit from Rule's latest venture, but said the writer made a contribution to the Susan Cox Powell Foundation, and any other money entitled to the family would be used to advance the foundation's mission.
"We don't have any desire to make any money on anything," Cox said, adding that Rule had started research on the book prior to the murder of his grandsons.
Rule could not be reached for comment, but told a Seattle radio station in the days following the Powell boys' slayings that she considers Josh Powell among the worst killers.
"This would be in the top five things that have shocked me over the last 30 years," Rule told radio station KIRO 97.3, in Washington. "A lot of parents kill their children to get revenge on an ex-spouse, but they usually don't die themselves."
Cox said he supports the idea of a book that would bring attention to the case and what may have gone wrong during the investigation into his daughter's 2009 disappearance, as well as shortcomings by Washington child welfare agencies which he believes emphasize the idea of family reunification over the safety of children.
Cox will tout that message Thursday during a press conference and town hall meeting in Olympia, Wash., where he'll appear with his attorneys and Washington state senator Pam Roach, a Republican who has demanded a review of child welfare policies following the death of the Powell boys.
Roach criticized thePowell boys' visitation with their father after concerns were raised that the boys were exposed to pornography at the home of their grandfather, Steve Powell.
Coxes grandchildren, Charlie, 7, and Braden, 5, died during a supervised visit with their father at a rental home. Josh Powell locked out the children's case worker out of the house before attacking his children with a hatchet and setting the home on fire with 10 gallons of gasoline. He left goodbye messages to family members saying he couldn't bear to live without his children, who had been placed into custody with the Coxes after Powell fell under investigation in a child pornography case involving his father.
Cox and Roach will join forces to see if the children's deaths can become a catalyst for change, so other families won't suffer the loss he and his family have endured.Cox said he's prepared to file a wrongful death lawsuit to facilitate change, but is optimistic he can make his points outside the legal system by working to revamp public policies.
"We're coming up with some things that should have been done differently," Cox said.
Cox said he doesn't blame any of the caseworkers involved in his grandson's case for what transpired and said they had the children's "best interest" at heart.
The Coxes are poised to keep up the work of the Susan Cox Powell Foundation, which formed in 2010 to give support to other families with missing loved ones.
Cox said he also hopes to work with other foundations dedicated to families who've gone through a tragedy.
"When people have a difficult situation, they'll know there is someplace to turn," Cox said. "I'm kind of excited about that, trying to make something good come out it."