"I'm not an internet person, but I realize that's the market of the future," he said, adding that if people didn't like his books they wouldn't have succeeded, no matter where and how much he talked about them. "I could give you a long list of people who have television and radio shows with books that didn't do well."
O'Reilly's departure has been among several for Fox over the past year, notably the ouster of founding CEO Roger Ailes. O'Reilly called his firing a "business decision" by Fox, where ratings have dropped in recent weeks. Nielsen says Fox's viewership in the 8 p.m. time slot that was O'Reilly's is down 13 percent between April and May.
"There's always cause and effect," he said. "We were doing extraordinarily well, bringing in audiences that were competitive with the networks. You take it out and there's going to be interesting things happening."
Multiple advertisers withdrew from O'Reilly's show before he left, and a similar pattern has developed for Fox host Sean Hannity, who has been strongly criticized for promoting a discredited story involving a murdered Democratic National Committee employee. O'Reilly thinks Hannity will stay on with Fox.
"It's the same thing, the far left going after him, trying to get him off the air," O'Reilly said. "I think Hannity will survive because I don't know if Fox can handle another shake-up like that."
O'Reilly said he chose the American Revolution because he had never read a book that explained it "top to bottom" and was also anxious to show the personal sides of George Washington and other leaders.
"You get to know all the people as people," he said during the recent telephone interview.
Holt has said all along that no changes were planned in his publication schedule, which includes three more "Killing" releases. Holt President and publisher Stephen Rubin has worked with O'Reilly on virtually all of his books and has defended him before, including when critics challenged the accuracy of "Killing Kennedy" and other works.
"We are totally committed to Bill, long-term. Why wouldn't we be?" Rubin said. "We have created the most successful adult nonfiction franchise in recent publishing history and we are thrilled to continue it."
According to Holt, the "Killing" series has more than 17 million copies in print.