In an angry, unsparing new book, Hillary Clinton said she regretted not striking back at James B. Comey in July 2016, when he accused her of acting recklessly in using a private email account while secretary of state, even as he announced that the F.B.I. would not seek criminal charges against her.

“My first instinct was that my campaign should hit back and explain that Mr. Comey had badly overstepped his bounds,” she wrote in the book, her attempt to make sense of her electoral defeat by Donald J. Trump. But, she said, her campaign advisers talked her out of it, convincing her that it was better “to just let it go and try to move on.”

“Looking back,” she concluded, “that was a mistake.”

It is one of a catalog of mistakes — large and small — that Mrs. Clinton lists in a post-mortem titled “What Happened.” The book, published by Simon & Schuster and scheduled to come out on Tuesday, has provoked a mix of anticipation and dread from Democrats, many of whom are loath to revisit the election that vaulted Mr. Trump into the White House.