When President Trump fired John Bolton as his national security adviser four months ago, I wrote “we may at last have in Bolton a piece of scorched debris from Trump’s inner circle disgruntled and disloyal enough to write a scathing tell-all memoir.” And so we’re about to have one—as well as his possible testimony, more damaging still, in Trump’s impeachment trial.

In anticipation of both, Trump and his team are denouncing Bolton as a revenge-seeking, money-grubbing malcontent. Trump himself has denied the headline-grabbing claim in Bolton’s book—that Trump told him he was holding up military aid to Ukraine until its president, Volodymyr Zelensky, agreed to investigate Joe and Hunter Biden. If true, this account would back up the impeachment charge passed by the House.

The New York Times, which described portions of the book’s manuscript, also reported that Bolton expressed concerns to Attorney General William Barr about Trump’s doing favors for the Chinese and Turkish presidents, Xi Jinping and Recep Tayyip Erdogan.

What to make of these head-spinning twists? Bolton has long been known as an ambitious, scheming bureaucratic operator. Are these claims in his book trustworthy? Or is he doing this simply to revive his profile and sell books?

I applauded Trump’s firing of Bolton, who was, by any measure, among the worst national security advisers in White House history, one of the very few who came to the job with a purely ideological agenda and, once there, wrecked the National Security Council apparatus—which conducted the analyses that usually inform interagency deliberation and decision-making—in order to push his biases. Trump encouraged his methods and, at first, supported his agenda—until he didn’t, and that was the end of him.

When it comes to following U.S. law and protocol, Bolton is a fairly straight arrow.

Still, I suspect that on the basic facts, Bolton’s account of his time with Trump—his book, due out March 17, is called The Room Where It Happened—is reliable. (I should note that Bolton and I share the same publisher, Simon & Schuster, but I have not seen any of Bolton’s manuscript or heard anything about its contents other than the Times story.) Here are a few reasons to believe it:

First, in his various stints in government, Bolton could be conniving and contentious. During the George W. Bush administration, he acted as a mole inside the State Department, informing Vice President Dick Cheney whenever Secretary of State Colin Powell was about to commit an act of arms control. He also tried to derail the careers of intelligence officials who disputed his views. Bolton was briefly ambassador to the United Nations, even while arguing that the United States should ignore international law. Yet amid all the fights he started and enemies he made in the Bush administration, he was not known to be an outright fabricator. And, as a lawyer who valued the written record as a tool of defense or intimidation, he always took very careful notes.

Second, disruptive as he could be, he has always regarded himself as a member of the foreign policy establishment. His goals could often be self-serving, but he saw them as aligned with the assertion of American power in the world. This is far from the same thing as Trump’s l’état, c’est moi style, in which U.S. interests are seen as synonymous with the interests of the Trump Organization. Bolton would certainly have been repelled by Trump’s striking personal deals with Xi and Erdogan—not because they’re dictators, but because Trump was subverting foreign policy for his own interests.

Third, during the House impeachment hearings, some officials testified that, when they told Bolton about Trump’s attempted quid pro quo with Ukraine, he told them to go talk to the NSC lawyer. He also was famously said to describe Rudy Giuliani’s schemes with corrupt Ukrainians as a “drug deal.” This is consistent with his claim in the book that he talked with Barr—the top law enforcement official—about Trump’s dealings with Xi and Erdogan. In other words, when it comes to following U.S. law and protocol, Bolton is a fairly straight arrow.

But what about the money? The revelations reported in the Times are sure to boost Bolton’s book sales. (The memoir has already reached No. 10 on Amazon’s best-seller list.) Then again, he is said to have received a $2 million advance to write this book. That’s an advance on royalties and, even if the book is a huge best-seller, he isn’t likely to make much beyond that advance. In any case, it’s worth noting that, when he signed the contract for that advance, nothing was known about Trump’s conversation with Zelensky. Whether or not he described it in his proposal, no one could have foreseen the huge splash that its revelation would make.

I have no idea whether Bolton’s book is good, whether it will be worth reading, whether the views it outlines on foreign policy in general and Trump in particular are laudatory or loathsome. (Much as I disagree with most of Bolton’s positions, I did take his side in his support of NATO, his grousing about Trump’s kowtowing to Russian President Vladimir Putin, and his skepticism of Trump’s trust in North Korean leader Kim Jong-un.) But if senators are wondering whether Bolton should be called to testify at the impeachment trial, and whether what he says under oath on the matters in question can be trusted, I’d say it’s worthwhile to issue that subpoena.

Update, Jan. 29, 2020: This piece was updated to clarify the Times’ reporting. According to its story, multiple people described the manuscript to Times reporters; the Times has not stated that it has a copy of the manuscript.