Katherine Brown was one of David O. Selznick’s most trusted employees. Her job—and it was a 24-hour-a-day one—was to scour New York’s publishing circles and sniff out unproduced properties that her hard-driving boss could spin into box-office gold. As the mercurial, 34-year-old movie mogul’s official East Coast story editor and unofficial gal Friday/arbiter of good taste, Brown needed to have lightning-quick instincts and the thick hide of an elephant. After all, Selznick was notoriously demanding and infamous for his short-fused three a.m. phone calls. It wasn’t easy keeping pace with a mind that whirred as feverishly as Selznick’s. But Brown was one of the few movie executives who could keep up.

On the afternoon of May 20, 1936, she fired off an urgent Teletype to her employer. The message read: “ . . . have just airmailed detailed synopsis of Gone with the Wind by Margaret Mitchell, also a copy of the book . . . This is an absolutely magnificent story . . . a great literary property and we must have it . . . The book is 1,000 pages long and I have only gotten through half of it, it is one of the most lush things I have ever read . . . I am absolutely off my nut about this book . . . I beg, urge, coax and plead with you to read it at once . . . I know that after you do you will drop everything and buy it.”

In fact, Selznick did not drop everything after reading Gone with the Wind. He didn’t even crack its imposing spine. Instead, he relied on the 150-page synopsis that Brown had given him, poring over it while on vacation in Honolulu. Selznick knew that the success of adapting Mitchell’s novel would depend on casting. Even though he was a compulsive gambler who lived to place exorbitantly large bets—whether in the boardroom or at the horse track—he balked at the book’s rich $50,000 price tag. But like all gamblers, he lived in a constant state of fear that someone else might rake in a pot that he felt rightfully belonged to him. He wasn’t sure he wanted Gone with the Wind, but he was certain that he didn’t want anyone else to have it.

So after several weeks of hemming and hawing, on July 6, 1936, Selznick finally relented to Brown’s impassioned pleas, opened his checkbook and paid a king’s ransom for the movie rights. It would be the easiest decision and the most incident-free moment of the next three years...