In the spring of 1974, after Robert A. Caro had finished writing The Power Broker: Robert Moses and the Fall of New York—a seven-year ordeal that took the book through three publishers and two editors and nearly bankrupted Caro—the first-time author got a surprise phone call from his agent, Lynn Nesbit.

“I submitted the book to The New Yorker,” she told him, “and Mr. Shawn [William Shawn, who was then the editor of The New Yorker] told me he's never read anything like it, and he's going to publish more of it than he's ever published of any book.”

Caro was stunned. A launch in The New Yorker guaranteed enormous national media attention for a biography concerned almost exclusively with the history of New York City. His publisher, Knopf, was equally excited and pushed back the publication of The Power Broker two months to give The New Yorker excerpts time to generate excitement around the book.

But the day he got Nesbit’s phone call—“That's the day my life changed,” Caro told The Daily Beast.

Of course, as with almost everything concerning the tortuous history of The Power Broker, it wasn’t quite that simple. According to William Whitworth, then the assignment editor at The New Yorker (and not coincidentally a Nesbit client), Shawn was initially put off by the size of the 1,296-page book. “He read it fairly quickly and said it was a wonderful piece of work, but we couldn’t do it,” Whitworth said. “It was just too long.”

Persuading Shawn to let him see what he could do with it, Whitworth spent six weeks paring the 650,000-word book into four 25,000-word pieces...