Mr. Branca hurt his back during spring training of 1952 and won just 12 games the rest of his career. He retired at 30 and lives in Westchester County, N.Y. He has long been a partner at an insurance and financial-planning company and today is chairman of the Baseball Assistance Team, which supports indigent former Major Leaguers.

“What’s it like to have to live with one awful moment?” wonders Mr. DeLillo in his novel. “Forever plodding across the outfield grass on your way to the clubhouse.”

A few years after surrendering baseball’s most famous home run, Mr. Branca heard talk that the Giants were stealing signs in 1951. “When I heard those rumors and innuendoes,” says Mr. Branca, now 75, “I made a decision not to speak about it.” He adds, “I didn’t want to look like I was crying over spilled milk.”

In 1962, an Associated Press article reported that a spy in the clubhouse helped the Giants win the pennant in 1951, but the story relied on an anonymous source and was vague. Soon after the story appeared, sportscaster Howard Cosell asked Mr. Branca to comment on it, but the pitcher demurred.

Over the years, as Messrs. Branca and Thomson rubbed elbows at countless functions, posed together with President Nixon and co-signed the sweet spots of baseball after baseball, the pitcher said nothing of sign stealing. “Bobby and I are really, really good friends,” Mr. Branca says. “He still hit the pitch.”