O'Reilly said the book would focus on World War II, and it will be released next September.
"It's a big World War II book with brand-new information about some very, very fascinating things, and we're writing it now," O'Reilly said.
But when asked if the title would keep with the "Killing," O'Reilly kept mum.
"I'm not tellin'," he said.
Police: Sword-wielding man demanded free tacos
Wisconsin business offering 'therapeutic cuddling' forced to close