Last week, Stephen Colbert giddily shared the most salacious details about President Donald Trump from Michael Wolff’s bombshell book Fire and Fury with his Late Show audience.

Monday night, he came face to face with the author.

“You know, he’s watching,” Wolff told Colbert of the president. As he explained, one of his remaining sources in the White House asked for the author’s media schedule. “So that might mean that all he’s done today is watch me on television.”

When Wolff said he was “surprised” by how much of a splash his book has made, Colbert said, “You describe the president as mentally unstable, unfit for the office, basically kind of jibbering to his cheeseburger when he goes to bed and he’s got the launch codes, why wouldn’t that cause a splash?”

“Because I thought we knew this,” Wolff said to cheers from the audience. “I kept saying, before this came out, to my publisher, I said, ‘Oh, you know, you’re printing a lot of copies, there’s nothing really too new in this book.’”

Now it’s the No. 1 book in the country and causing the president to lose his mind.

Colbert also confronted Wolff about some of the discrepancies in his book—ones the critics have used to smear him as a “fiction” writer. In the foreword, Wolff says that he aimed to depict “the events I believe to be true,” as opposed to the absolute truth. “What does that mean?” Colbert wanted to know.

“It’s the Trump White House,” Wolff explained. “Everybody is telling you different stories. Let’s put it this way: Everybody is lying in their own particular way because that’s what you do in the Trump White House. So I had to go and take whatever the event was, find as many people as I could, and then use my judgment.”

Colbert told Wolff that he was “deeply conflicted” reading the book. As a comedian, he wants the stories to be true so that he can make jokes about them, but as a citizen, he said he hopes they are exaggerated. “And you don’t have sourcing, at least not listed, for everything you’ve got in here, so how much should I believe?”

“You should believe all of it,” Wolff said. “That’s the alarming thing, that this is all true.”

When Colbert asked him why he won’t release the recordings he took so he can “slap down the character attacks against [him] by the White House,” Wolff replied, “Because I’m not in the recording—I’m in the writing business.”

“I’m offering something different,” he continued. “I’m offering—and this was totally mystifying to people in the White House—I’m offering a book.” He said people should read it and ask themselves, “‘Does this comport with what you already know? Does it make sense? Does it have an internal integrity in which you come away saying, I think I understand this now?’ That’s my job as the writer.”

Asked if there was anything that he saw in Trump’s White House that gave him “hope,” Wolff took a long pause and said, “Nothing.”