“I deal with a lot of those issues at length in the book and explain the context and circumstances,” Mr. Spicer said, including the issue of lying: “You may not agree with what I say, but you might say, ‘That’s an interesting explanation.’”

Post-White House, Mr. Spicer has landed a spot as an unpaid pundit on Fox News and his representatives are seeking a network to buy a pilot talk show, tentatively named “Sean Spicer’s Common Ground,” in which he would meet “some of the most interesting and thoughtful public figures for a drink and some lite conversation at a local pub or cafe,” the pitch sheet says.

He has recorded three episodes of a podcast with the conservative writer Katie Pavlich called “Everything’s Going to Be All Right,” and will record a fourth from Tuesday’s book party, before taking a hiatus until after the book tour. Mr. Spicer is also a spokesman and senior adviser for America First Action, a pro-Trump “super PAC” that has struggled to reach its fund-raising goals for November’s congressional midterm elections.

What does he aim to accomplish with his book?

“Before I answer this question, let me pause and think what I’ve agreed to with other papers,” he said, before beginning to draft a statement out loud. “It hopefully answers a lot of questions as to who I am, how I got to where I did, and to walk through some of the transitional moments of the campaign, transition and the early days of this White House. Wait, instead of ‘the White House,’ let’s say ‘the early days of the administration,’ since ‘the White House’ sounds more like a building.”

In an advance copy of “The Briefing” obtained by The Guardian, Mr. Spicer describes his former press secretary job as requiring the skills of a fighter jet pilot, champion boxer and tightrope artist. Mr. Spicer pointed out that he did not make all those comparisons in the same sentence.

The book also lauds Mr. Trump, whose “furious assault on his opponents is a talent few politicians can muster.”

The president seems pleased. He tweeted an endorsement of “The Briefing” to his 50 million-plus Twitter followers on a recent Saturday, saying of the book, “Really good, go get it!”