A new book is reportedly offering more details on former Defense Secretary James Mattis' alleged disagreements with President Trump.

Set for release on Oct. 29, the book --"Holding the Line: Inside Trump’s Pentagon With Secretary Mattis" -- claims that Mattis dreaded seeing Trump's military parade and at one point said the White House was "undisciplined" and "not to be trusted," The Washington Post reported on Wednesday.

Mattis was so averse to Trump's proposed, multi-million dollar parade that he said he would rather "swallow acid" than watch it -- according to the new book. Shortly after The Post's report was published, Mattis issued a statement taking issue with the book's author. "His choice to write a book reveals an absence of character," Mattis' statement read, before continuing: "He may receive a few brief moments of attention for this book, but those moments will be greatly outweighed by the fact that to get them, he surrendered his honor."

The parade, scheduled for Veteran's Day in 2018, was eventually canceled by Trump who cited a "ridiculously high" price.

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The book's author, retired Navy commander Guy Snodgrass, previously served as Mattis' speechwriter and director of communications. He claims that Mattis wanted to leave prior to December of 2018 but used a disagreement about Syria as a "pretext to announce a decision that he had made months before to cut his losses and move on."

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Trump wanted to pull out of Syria, something he eventually did and was criticized for, given Turkey's subsequent incursion.

The Post said the book also offered more details surrounding a $10 billion contract that the Pentagon delayed awarding after Trump expressed concern over Amazon Web Services, which would likely have won the contract.

According to Snodgrass, Trump wanted to lock the company out of the bid as a way to "screw" Amazon CEO Jeff Bezos. Mattis reportedly responded by indicating that was illegal and unethical.

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“We’re not going to do that,” Mattis reportedly told senior Pentagon officials. "This will be done by the book, both legally and ethically.”

Since leaving office, Mattis has mostly refrained from attacking the president. "I will not speak ill of a sitting president. I'm not going to do it," Mattis previously told CBS upon releasing a book of his own.

"He's an unusual president, our president is. And I think that especially with, just the rabid nature of politics today, we gotta be careful. We could tear this country apart."

On his decision to leave the administration, Mattis told CBS: “When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution."

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During a meeting with members of Congress in October, Trump dismissed Mattis as "the world's most overrated general." Mattis responded by joking about it during a charity dinner on Thursday.

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"I'm not just an overrated general. I'm the greatest, the world's most overrated," he told diners at the annual Alfred E. Smith Memorial Foundation Dinner.

"I'm honored to be considered that by Donald Trump because he also called Meryl Streep an overrated actress," he said. "So I guess I'm the Meryl Streep of generals, and frankly that sounds pretty good to me."

The Associated Press contributed to this report.