On Tuesday morning, when Attorney General William Barr finally appears before the House Judiciary Committee, a book will be released covering one of Barr’s most controversial and most consequential actions to date: the attorney general’s grossly misleading summary of the Mueller Report.

The book’s author is Ambassador Norman Eisen, who served as special counsel to the Judiciary Committee during the impeachment hearings of Donald J. Trump. His was not simply a ringside seat; Eisen was a key player. That’s why this behind-the-scenes account sheds new light on the history-shaping impact of Barr’s actions.

“He’s lying.” Chairman of the House Judiciary Committee Jerrold Nadler privately told Eisen and other staff as they tried to make sense of Barr’s 4-page summary without the benefit of the Mueller Report itself. The Chairman “saw right through Barr’s fabrications and was blunt about it,” writes Eisen in the book.

Not everyone else did. Former FBI Director James Comey said in a CNN interview at the time that “Bill Barr, our attorney general, deserves the benefit of the doubt.”

Nadler would be proven correct once the Mueller Report was released, but that would be more than three weeks later—a lifetime in American politics. And it would be almost an exact year before a federal court would weigh in. Judge Reggie B. Walton used part of his opinion in March of this year to call out Barr for the attorney general’s “misleading” and “distorted” account of Mueller’s findings.

That was a harsh assessment with added weight due to its legal significance. As Lisa Gilbert observed at Just Security, “To underscore the significance of Judge Walton’s findings: Barr’s summary of the Mueller Report was not simply a lie told to the media or public. It was a statement Barr submitted to Congress.”

However distorted Barr’s summary was, some would say the public still got the benefit of the Mueller Report to decide for itself on the president’s conduct. Jack Goldsmith appears to have held that view. He wrote approvingly of Barr’s telling Congress, “‘The report is now in the hands of the American people. Everyone can decide for themselves.’” “Thanks to Mueller’s hard work and Barr’s decision to publish the Mueller report nearly in full, the Congress and the American people, and the ‘democratic process,’ can now judge the president,” wrote Goldsmith in May of last year.

Eisen disagrees. He responded to that idea in an interview with Just Security:

“As I explain in the book, the Attorney General’s out-and-out lies in his summary of the Mueller Report had a devastating effect on Congress’s work. That is because he dropped those initial revelations of the actual contents of the report into a vast and long-unfed hunger for Mueller’s opinions, and the Attorney General’s falsehoods were taken up as the truth. My book talks about how we tried to fight this, but failed, because at that point, no one believed that the Attorney General would be willing to sacrifice his reputation for the sake of Trump. Now, over a year and many many lies later, we know much better. At any rate, the Attorney General’s spin job galvanized the collective wisdom around the idea that Mueller had let Trump off, and it drained the energy out of the Report. Later, when we got the Report and saw the shocking evidence of criminal misconduct, it was too late to recover that public and political energy. To my dying day, I will believe that if the redacted Mueller Report had been released without Barr’s intervention, we would certainly have seen a much earlier impeachment against Trump based on at least five episodes of obstruction of justice. We certainly would have seen freestanding articles on obstruction as part of the ultimate Ukraine articles, which we came very close to getting anyhow.”

Eisen’s book details the havoc Barr’s summary wrought inside the halls of Congress. It will be a powerful moment to see the attorney general now come before the House Judiciary Committee. He should be asked not only whether he stands by the summary he submitted directly to Congress. He should also be asked whether he defends other statements he made to the public about the Mueller investigation that were also, with the passage of time, proven false.

One of Mueller’s lead prosecutors, Andrew Weissmann, recently identified one of those howlers. Attorney General Barr stated in prepared remarks at a press conference prior to the release of the Mueller report, “The White House fully cooperated with the Special Counsel’s investigation.” Weissmann framed the right question for Congress to ask in a tweet in the run-up to Tuesday’s hearing:

On Tuesday, members of Congress would do well to ask Barr whether he testifies—now under penalty of perjury—that his statement about the president’s “full” cooperation is true. There are a host of other public statements by Barr and the Justice Department’s spokesperson that should be subject to the same line of question (“Do you hereby testify under penalty of perjury that a public statement you/the Justice Department spokesperson made is accurate and true?”).

In the end, the attorney general may suffer no great personal consequences for lying to Congress. That’s not due to a gap in the law, but simply a political reality. Regardless, the record has become clearer how much he has, indeed, lied and the far-reaching consequences of that behavior are plain to see. Eisen’s book is an important piece of the historical record. A congressional airing of these matters is another. How much credibility Barr has left is important. It’s the attorney general’s credibility that will determine how effective he’ll be in misleading the American public on other matters as he works toward President’s Trump’s reelection.