In the book, Comey describes Trump as being "untethered to the truth" and likens him to a mob boss and a bully.
Comey also writes that Trump continued to ask him about unverified, salacious allegations that there was tape of him watching prostitutes urinate on each other in a Moscow hotel room in 2013. The former FBI director said Trump denied the lurid tale, but said he might order the FBI to investigate the claims.
In an interview with ABC News, Comey said he didn't know whether he believes Trump's denials. A snippet aired Friday morning, and a longer version of the interview is slated to be broadcast 10 p.m. ET on Sunday.
Trump fired Comey last May, four months after the Justice Department veteran first briefed him on the allegations about the prostitutes. The president told NBC News' Lester Holt that he had been thinking about the probe into Russian interference in the 2016 election when he decided to fire Comey.
Weeks later, Deputy Attorney General Rod Rosenstein appointed former FBI Director Robert Mueller, Comey's predecessor at the FBI, to take over the Russia probe as special counsel. Mueller is reportedly investigating Trump for obstruction of justice and possible collusion with the Russians.
Trump has repeatedly denied accusations of collusion and obstruction.