It was Trump who demanded allegiance from Comey early this year, before firing him in frustration over the FBI's mounting probe into whether his campaign colluded with Russian meddling in the 2016 election.
"I need loyalty, I expect loyalty," Trump told the country's top investigator at a private White House dinner in January, one week after Trump became president, according to Comey.
In a memorandum he wrote after the dinner, Comey said he was uncomfortable with Trump's repeated entreaties, and had only promised "honest loyalty" afterward.
But that backfired: it sparked the appointment of a special prosecutor for the investigation, Robert Mueller, a former FBI director, whose mandate then expanded to include possible obstruction of justice by the president himself.
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Days after the firing, furious that Comey had kept and leaked memos of their discussions, Trump blasted via Twitter:
"James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!"
To which Comey replied, in Congressional testimony weeks later, "Lordy, I hope there are tapes."
The publisher added nothing to that promise today, although it sounded like Trump could feature prominently. Flatiron said the book, to be released in May 2018, will "explore what good, ethical leadership looks like and how it drives sound decisions."
On his own rarely used Twitter account -- @formerBu -- Comey simply tweeted: "Lordy I hope there are pictures.