The book deal with Simon & Schuster was first reported by The Associated Press on Saturday. And it comes at a time when Bolton is a much-sought-after witness in the impeachment inquiry. He is refusing to testify until a federal judge reaches a decision in the legal fight over claims of immunity for White House officials.

Bolton’s lawyer said in a letter on Friday that Bolton “was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far.”

A source close to Bolton recently told CNN that if Bolton does ultimately end up testifying, he will say that Mulvaney and others, including the president’s personal lawyer Giuliani, “were being disloyal to the president” in their actions relating to the Ukraine scandal.