Bill O'Reilly said he is working on the fourth book in the series that began with "Killing Lincoln," "Killing Kennedy" and "Killing Jesus."
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.