Trump, who expects loyalty from his staff members — even the ones he has fired — responded by claiming Bannon had "lost his mind".
It's a colossal, raging dumpster fire of animosity, made all the worse by the spiteful personalities of both men.
Meanwhile, the book in question, written by Michael Wolff, is soon to be released in the United States.
Given how juicy the excerpts have been so far, we can expect it to contain even more politically explosive claims, so Trump's mood is unlikely to improve.
But the President has bigger things to agonise over. I'm not even talking about, say, North Korea, immigration, the national debt or any policy issue for that matter.
No, on a purely political level, something happened this week that could cause Trump far more pain in the long run than a mere book — and drive an even deeper wedge down the middle of the Republican Party.
A really old guy retired.
I'm talking about Orrin Hatch, a rather dull and nondescript 83-year-old who has hogged a seat in the US Senate since the 1970s. If you haven't heard of him, it's probably because you have a life.
Trump had publicly pushed Hatch to pursue yet another term, partly because he was a reliable Trump supporter, but more importantly, because the man likely to replace him in the Senate could become an even bigger pain in the backside than Bannon has proven to be.
That man is Mitt Romney.
This time the name probably should ring a bell, because Romney was the Republican nominee for president whom Barack Obama vanquished to win re-election in 2012.
With Hatch gone, Romney is reportedly preparing to run for the now vacant Senate seat, and polls show he can expect to win comfortably with about 70 per cent of the vote.
"So what?" I can hear you thinking. Why should Donald Trump care if some guy who failed to become president makes it into the Senate?
Because the already fragile unity of his party would be threatened.
Romney has been among Trump's most vocal critics on the conservative side of politics.
During the 2016 election campaign, he called Trump a "phony" and "fraud" who was unworthy of his vote.
Since then, he has continued to slap down Trump over some of his more outlandish behaviour in office. And as a former presidential nominee, Romney is still a man of considerable stature and influence in the Republican Party.
He also happens to be everything Trump is not, for better or worse. Where the president is endlessly entertaining, Romney is quite boring. But where Trump is frequently vulgar and, let's be honest, not exactly known for his moral principles, Romney — an earnest, absurdly straight-laced Mormon who still uses words like "golly" and "gee whiz" — is of unimpeachable character.
He would probably support most of Trump's agenda in Congress, but that isn't really the point here.
This is about Trump's tone and style of leadership. It's about the battle between the nationalist right and more traditional conservatism — what Trump calls "the establishment".
Romney's elevation to the Senate would give Trump's currently rudderless Republican critics a real figurehead, with the name recognition and authority to go toe-to-toe with the President. It would reignite the war to define conservatism in the United States — a war Trump thought he had already won.
"Should he run and win Utah's open Senate seat, Mr Romney will likely find himself the leader of the #NeverTrump right, a faction that has been riven by infighting," writes Will Rahn, a commentator for CBS News.
"If all goes well, he could even position himself as a sort of alternate reality Republican president."
The last thing Trump wants or needs is an "alternative president". He is already surrounded by foes — the Democrats oppose his every move, the alt-right faction represented by Bannon is looking shaky, the media is always hounding him and the special counsel investigation of Robert Mueller is still working quietly behind the scenes.
Under constant attack from multiple directions, Trump can't afford a revolt from within his own party.
That is the threat Romney poses.
It is why Trump tried to stop Orrin Hatch from retiring.
And it is ultimately far more important than trading puerile insults with a disgruntled former staffer.
