John Bolton lands a book deal. It will publish before the 2020 presidential election

New York (CNN Business) John Bolton has stayed pretty quiet since resigning as President Trump's national security adviser in September. But he evidently has a book's worth of something to say.

Bolton has struck a deal to write a book for Simon & Schuster, according to two sources with knowledge of the deal.

One of the sources said the book will come out sometime in 2020 before the presidential election.

The publishing house declined to comment on Saturday, as did a spokesperson for Bolton.

Bolton was previously reported to be in talks about a potential book project -- which sparked speculation about whether he would publicly break with Trump.

And Bolton's relationship with Trump soured in the final months of his eighteen-month-tenure, over both substantive policy disagreements and Bolton's sharp-elbowed approach to his job.

The former UN ambassador frequently clashed with other senior members of Trump's team, including acting White House chief of staff Mick Mulvaney, leaving him with few allies when his relationship with Trump began to collapse.

On the day Bolton's exit was announced, Trump said he "strongly disagreed with many" of Bolton's suggestions "as did others in the administration."

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.

"John will say, 'I was the guardrail while Giuliani and Mulvaney were saying let Trump be Trump,'" the source said.