Explaining a lack of testimony, and placing blame on Democrats.

As the book nears publication and details spill out, many congressional Democrats quickly assailed Mr. Bolton for not telling his story during the impeachment proceedings and instead saving it for his $2 million book.

Mr. Bolton explains his position in the epilogue, saying he wanted to wait to see if a judge would order his former deputy to testify over White House objections. House Democrats opted not to pursue the case, fearing endless litigation. Once the House impeached Mr. Trump over the Ukraine matter, Mr. Bolton volunteered to testify in the Senate trial that followed if subpoenaed.

But Senate Republicans voted to block new testimony by him and any other witnesses even after The New York Times reported that his forthcoming book would confirm the quid pro quo. Some of those Republican senators said that even if Mr. Bolton was correct, it would not be enough in their minds to justify making Mr. Trump the first president in American history convicted and removed from office.

Mr. Bolton blames House Democrats for being in a rush rather than waiting for the court system to rule on whether witnesses like him should testify, and he faults them for narrowing their inquiry to just the Ukraine matter rather than building a broader case with more examples of misconduct by the president.

“Had a Senate majority agreed to call witnesses and had I testified, I am convinced, given the environment then existing because of the House’s impeachment malpractice, that it would have made no significant difference in the Senate outcome,” he writes.

Singling out episodes of “obstruction of justice as a way of life.”

The other episodes that Mr. Bolton says the House should have investigated include Mr. Trump’s willingness to intervene in Justice Department investigations against foreign companies to “give personal favors to dictators he liked.” Mr. Bolton said it appeared to be “obstruction of justice as a way of life.”

He singles out Halkbank of Turkey, a state-owned financial institution investigated for a multibillion-dollar scheme to evade American sanctions on Iran. At a side encounter during a Buenos Aires summit meeting in late 2018, President Recep Tayyip Erdogan of Turkey handed Mr. Trump a memo by the law firm representing Halkbank, “which Trump did nothing more than flip through before declaring he believed Halkbank was totally innocent.” He then told Mr. Erdogan “he would take care of things.”

Attorney General William P. Barr later spent months trying to negotiate a settlement with the bank, but that came to an end in October, after Mr. Bolton left office, when the Justice Department charged Halkbank in a six-count indictment.

Mr. Bolton also mentions ZTE, the Chinese telecommunications giant that was convicted of evading sanctions on Iran and North Korea and then faced new penalties for further violations during its follow-up consent decree. During a conversation on trade with President Xi Jinping of China, Mr. Trump offered to lighten the penalties.

“Xi replied that if that were done, he would owe Trump a favor and Trump immediately responded he was doing this because of Xi,” Mr. Bolton writes. He called himself “appalled” and “stunned” by the idea of intervening in a criminal investigation to let a sanctions buster off the hook. In the end, at Mr. Trump’s behest, the Justice Department accepted a $1 billion fine and lifted a seven-year ban on buying American products, an act of lenience that saved the company from going out of business.

A new allegation in the book accuses Mr. Trump of “pleading” with Mr. Xi to help him win re-election by buying American agricultural products, which would help the president in farm states. Mr. Trump did not deny it when asked about the matter on Wednesday night by Sean Hannity on Fox News, but Robert Lighthizer, his trade representative, did on his behalf earlier in the day, saying it was not true.

Describing a toxic environment inside the administration.

Over a long career in and out of Republican administrations in Washington, Mr. Bolton has rarely shied from giving his opinions, usually born of strong conservative national security convictions that have made him one of the capital’s most outspoken hawks advocating the use of military power and sanctions.

While he agreed with Mr. Trump on issues like getting out of the nuclear accord with Iran, he found himself repeatedly trying to stop the president from making concessions to other rogue states or making an ill-considered peace deal with the Taliban in Afghanistan while pushing for a more robust use of force against outliers like Iran or Syria. He considered Mr. Trump’s diplomacy to be folly.

To Mr. Bolton, Mr. Trump’s decision to meet North Korea’s leader, Kim Jong-un, in Singapore was a “foolish mistake,” and the president’s desire to then invite Mr. Kim to the White House was “a potential disaster of enormous magnitude.” A series of presidential Twitter posts about China and North Korea were “mostly laughable.” Mr. Trump’s meeting with President Vladimir V. Putin of Russia in Helsinki was a “self-inflicted wound” and “Putin had to be laughing uproariously at what he had gotten away with in Helsinki.”

Mr. Bolton also describes an environment inside the administration marked by caustic infighting in which various players trash one another in a contest for the president’s ear — and the president trashes all of them.

When Mr. Bolton took over as national security adviser in 2018, John F. Kelly, then the White House chief of staff, disparaged the departing adviser, H.R. McMaster, by saying, “The president hasn’t had a national security adviser in the past year and he needs one.” Mr. Pompeo, the book says, disparaged Nikki R. Haley, then the ambassador to the United Nations, calling her “light as a feather.”

Battling over what is deemed classified information.

The Justice Department has gone to court to stop the book from being published, arguing that it has classified information in it and that it was not cleared by a prepublication review required of former government officials like Mr. Bolton.

In fact, according to his lawyer, Charles J. Cooper, Mr. Bolton participated in an extensive back-and-forth over the book and agreed to all of the revisions mandated by the career official who reviewed it or came up with acceptable alternatives. Only when the review was over did another official, Michael J. Ellis, a political appointee, step in to review it all over again at the instruction of Robert C. O’Brien, Mr. Bolton’s successor as national security adviser.

If there is classified information still in the book, it is hard to figure out what it might be. There are not references to secret intelligence programs or espionage sources and methods. But Mr. Trump insisted this week that every conversation with him was “highly classified” and therefore could not be disclosed, an assertion that goes far beyond tradition.

In his epilogue, Mr. Bolton says that in a few cases, “I was prevented from conveying information that I thought was not properly classifiable, since it revealed information that can only be described as embarrassing to Trump or as indicative of possible impermissible behavior.” One example is the direct quote of what Mr. Trump said to Mr. Xi about helping him win re-election.

For the most part, though, Mr. Bolton explains in the epilogue that the career official who reviewed the book merely made him take quotation marks off things that the president said and otherwise generally left them in. And so Mr. Bolton offers a guide to readers: “In some cases, just put your own quotation marks around the relevant passages; you won’t go far wrong.”