The focus will be on past Democratic criticism of Comey. News of his letter to Congress splashed in the headlines as early voting proceeded.
While Comey doesn't divulge the information, many speculate whether the document relates to Lynch's infamous "tarmac meeting" with former President Bill Clinton just days before her Justice Department exonerated Hillary Clinton of any wrongdoing.
Trump, who criticized Comey repeatedly before and after he fired him, denounced him on Twitter on Friday: "James Comey is a proven LEAKER & LIAR", he said. While stopping short of regret, Comey does admit faults in his conduct of the affair.
Clinton herself has cited Comey's handling of the email probe as a reason she lost the election.
Yet in his book, Comey admits that at the July 2016 news conference, in which he spoke for more than 10 minutes about Clinton's poor email practices before announcing that the Federal Bureau of Investigation would not recommend that she be prosecuted, he might have buried the lede.
"I believed it was my duty to inform Congress that we were restarting the investigation", he writes about the October 2016 statement.
"Although we did not find clear evidence that Secretary Clinton or her colleagues meant to violate laws governing the handling of classified information, there is evidence that they were extremely careless in their handling of very sensitive, highly classified information", he said at the time.
"I don't remember knowingly thinking about that, but it has to have been because I had been operating in a universe where by Hillary Clinton was going to overcome Donald Trump, and thus I'm pretty convinced it had been a factor", Comey explained in a excerpt of an interview aired on Saturday.
Comey was anxious that the country's top law enforcement agencies would suffer if the public later discovered the Federal Bureau of Investigation had obtained new emails in the Clinton probe. He also says that he wouldn't have changed his broader conduct, "but I can imagine good and principled people in my shows making different choices about some things".
ABC News reports, "Comey writes that he felt obligated to take more of a personal role as the public face of the investigation rather than deferring to then-Attorney General Loretta Lynch - in part because of something involving Lynch that he cryptically refers to as a 'development still unknown to the American public to this day'".