Mark Pomerantz is a well-credentialed former federal prosecutor. As a younger man he clerked for a supreme court justice and helped send the mob boss John Gotti to prison. He did stints in corporate law. In 2021, he left retirement to join the investigation of Donald Trump by the Manhattan district attorney. Pomerantz’s time with the DA was substantive but controversial.

In summer 2021, he helped deliver an indictment for tax fraud against the Trump Organization and Alan Weisselberg, its chief financial officer. At the time, Cy Vance Jr, the son of Jimmy Carter’s secretary of state, was Manhattan DA. Pomerantz also interviewed Michael Cohen, Trump fanboy turned convicted nemesis, pored over documents and clamored for the indictment of the former president on racketeering charges.

For Pomerantz, nailing Trump for his hush money payment to Stormy Daniels, the adult film star who claims an affair Trump denies, didn’t pass muster. But that avenue of prosecution was a “zombie case” that wouldn’t die. It still hasn’t: a Manhattan grand jury again hears evidence.

Pomerantz saw Trump as a criminal mastermind aided by flunkies and enforcers. He believed charges ought to align with the gravity of the crimes. But as Pomerantz now repeatedly writes in his memoir, Alvin Bragg, elected district attorney in November 2021, did not want to move against Trump. In early 2022, Bragg balked. In March, Pomerantz quit – and leaked his resignation letter.

“I believe that Donald Trump is guilty of numerous felony violations of the penal law,” Pomerantz fumed. “I fear that your decision means that Mr Trump will not be held fully accountable for his crimes.”

Now comes the memoir, People vs Donald Trump: An Inside Account. It is a 300-page exercise in score-settling and scorn. Pomerantz loathes Trump and holds Bragg in less than high regard. He equates the former president with Gotti and all but dismisses the DA as a progressive politician, not an actual crime-fighter.

In a city forever plagued by crime and political fights about it, Bragg’s time as DA has proved controversial: over guns, trespassing, turnstile jumping, marijuana and, yes, the squeegee men.

Bragg is African American. This week, a group of high-ranking Black officials protested against Pomerantz’s attacks. In response, Pomerantz called Bragg “respected, courageous, ethical and thoughtful” but said: “I disagreed with him about the decision he made in the Trump case.”

In his resignation letter, Pomerantz wrote: “I have worked too hard as a lawyer, and for too long, now to become a passive participant in what I believe to be a grave failure of justice.”

Trump, he now writes, “seemed always to stay one step ahead of the law”. That may conjure up images of Road Runner and Wile E Coyote but Pomerantz is serious. “In my career as a lawyer, I had encountered only one other person who touched all of these bases: John Gotti, the head of the Gambino organised crime family.”

The Goodfellas vibe is integral to Trumpworld. In The Devil’s Bargain, way back in 2017, Joshua Green narrated how Trump tore into Paul Manafort, his then campaign manager, shouting: “You treat me like a baby! Am I like a baby to you … Am I a fucking baby, Paul?” It was if Trump was channeling Joe Pesci.

With the benefit of hindsight, Pomerantz concludes that the US justice department is better suited to handle a wholesale financial investigation of Trump than the Manhattan DA. Then again, the attorney general, Merrick Garland, has a lot on his plate. An insurrection is plenty.

Pomerantz’s book has evoked strong reactions. Trump is enraged, of course. On Truth Social, he wrote: “Crooked Hillary Clinton’s lawyer [Pomerantz says he has never met her], radically deranged Mark Pomerantz, led the fake investigation into me and my business at the Manhattan DA’s Office and quit because DA Bragg, rightfully, wanted to drop the ‘weak’ and ‘fatally flawed’ case. This is disgraceful conduct by Pomerantz, especially since, as always, I’ve done nothing wrong!”

Really?

In December, a Manhattan jury convicted the Trump Organization on 17 counts of tax fraud and the judge imposed a $1.6m fine. Alan Weisselberg pleaded guilty and testified against his employer. Trump and three of his children – Ivanka, Don Jr and Eric – are defendants in a $250m civil lawsuit brought by Letitia James, the New York attorney general, on fraud-related charges. That case comes to trial in October 2023, months before the presidential primary. Sooner than that will be the E Jean Carroll trial, over alleged defamation and a rape claim Trump denies.

Significantly, state prosecutors say Pomerantz may have crossed an ethical line.

“By writing and releasing a book in the midst of an ongoing case, the author is upending the norms and ethics of prosecutorial conduct and is potentially in violation of New York criminal law,” J Anthony Jordan, president of the District Attorneys Association of the State of New York, announced.

Bragg accused Pomerantz of violating a confidentiality agreement. Pomerantz is unbowed. “I am comfortable that this book will not prejudice any investigation or prosecution of Donald Trump,” he states on the page. No formal ethics complaint has appeared.

Pomerantz also offers a window on personalities that crossed his path. Cohen receives ample attention. Pomerantz lauds Trump’s former fixer for his cooperation but reiterates that Cohen pleaded guilty to perjury.

His conduct left Pomerantz shaking his head. Cohen’s liking for publicity could be unsettling. So was his Oval Office tête-a-tête with Trump over the payment to Daniels. Pomerantz was disgusted. Trump and Cohen, he writes, defiled America’s Holy of Holies, its “sanctum sanctorum”.

No harm, no foul. Cohen’s lawyer, Lanny Davis, announced: “Mr Cohen will continue to cooperate with DA Bragg and his team, speaking truth to power – as he has always done.” On Wednesday, Cohen met the Manhattan DA for the 15th time. Pomerantz is gone. The show goes on.

• People vs Donald Trump: An Inside Account is published in the US by Simon & Schuster