Over my many years writing editorials about New York for The New York Times, the city’s political class had a preferred list of adjectives for then mayor Michael Bloomberg: staid, stoic, irritable, irritating, distant, insensitive, gray—or any synonym for boring. Bloomberg can indeed earn all of those descriptions, I found, as I produced an unauthorized biography of this powerful billionaire. But underneath the cardboard, he is a complicated man who can be funny, smart, raunchy, generous, gutsy (an early foe of the N.R.A.), persuasive, and hell-bent on being the kind of better billionaire that Donald Trump isn’t.

At heart, Bloomberg is also a salesman who wants you to buy into his view of the world (or, failing that, what about one of his Bloomberg data terminals at $24,000 a year?). A trained engineer, he wants to fix things, even a gun-plagued America and a warming planet. He’s announced plans to spend $500 million fighting climate change and even more on the elections next year. At about one percent of his net worth, that is almost certainly a down payment.

Unlike you-know-who in Washington, Bloomberg is not your average flashy tycoon. But he really survives on work and chafes at empty hours on his calendar. His sartorial style is old-world comfort, the basic uniform of business. He likes shoes so worn and patched that he once gave a strong recommendation for “Saul, the sole man,” whose shoe-repair shop in Lower Manhattan I could never find.

His dinners are like college seminars. And when he built a $1.6 billion European headquarters in London, he told the starchitect Norman Foster that he wanted his office building to be green and “classy”—not another phallic tower in London, taller than the next guy’s.

Unlike some of his gaudier peers, Bloomberg is also secretive about his wealth. Every day, his company features a list of the richest people in the world. His name is not on it.

Bloomberg is secretive about his wealth. Every day, his company features a list of the richest people in the world. His name is not on it.

That is an old habit, apparently. When he became a public figure in 2001, Bloomberg put out vague renderings of his tax returns, and when, in 2015, I went to the city’s Conflicts of Interest office looking for more Bloomberg data, an official informed me that they only preserved their reports for six years. Nothing for the first six years in office? Nothing about how he made an agreement to step away from his company (except in four major instances, like if somebody tried to sell Bloomberg LP)? Furious, I nagged one official about where to find these historic documents.

“Try Wayne Barrett’s basement,” he said finally.

Wayne Barrett’s basement was almost as legendary as Wayne Barrett. A superb investigative reporter who died in 2017, Barrett was generous with other reporters willing to attack the terrifying mass of boxes clogging the basement of his Brooklyn home. After my one feeble attempt, the freelance writer John Surico unearthed a stack of folders full of the missing Conflicts of Interest Board reports. They were a beautiful sight.

In 2002, Bloomberg owned 95 stocks, 99 bonds or bond funds, and five homes worth more than $500,000, the city’s largest category. His ex-wife, family, and a former girlfriend fared well, and he had invested $10 million in a film based on a novel by Arthur Miller about a man who gets glasses and learns about anti-Semitism. He owned 80 percent of Bloomberg LP, and Forbes estimated his worth that year at around $4.8 billion, a number that has grown steadily to today’s $55.5 billion.

How he uses that remaining fortune is the next chapter in Bloomberg’s high-powered life.

Eleanor Randolph was a writer on The New York Times’s editorial board until 2018. Her book will be published by Simon & Schuster on September 10