Before the comic-book line edited by Stan Lee was even called Marvel Comics, it thrived on feedback from its young readers. When I set out to read all 27,000 Marvel superhero comic books published from 1961 onward, for my book All of the Marvels, I didn’t realize how fascinating old comics’ letters pages would be as historical artifacts.

The first letter column to appear in one of the company’s 60s superhero titles—1962’s Fantastic Four No. 3—was partly a plant: the “S. Brodsky” credited with one of its letters was Sol Brodsky, who had inked that issue. At least some of it was for real, though: 13-year-old Alan Weiss, who also had a letter printed in that issue, went on to draw comics for Marvel in the mid-1970s.

Stan Lee, head of Marvel, in his New York office, 1975.

Lee replied to all his correspondents with a tone whose manic chumminess disguised his genuine desire to encourage. As I read through those early issues, I recognized one name after another among the “letterhacks,” as they were known, including Roy Thomas (who would eventually become Lee’s successor as Marvel’s editor in chief), Buddy Saunders (who subsequently founded mail-order behemoth Lone Star Comics), Dave Cockrum (who went on to draw the landmark 70s revival of X-Men), and even the young George R. Martin, who had yet to add the second R to his name.

Covers for Millie the Model, Marvel’s longest-running humor title (1945–73), and The Amazing Spider-Man, created by Lee.

The letters printed in Marvel’s comics offered intimations of things to come as well. One correspondent in 1968’s Doctor Strange No. 169 suggested that Marvel might “take some of [their] very best serials and bind them into handsome, hardcover editions on glossy paper,” long before that became a reality. Richard and Wendy Pini, the co-creators of Elfquest, first met through the letter pages of Silver Surfer.

In the 1970s, Marvel’s letter columns became arenas for broader conversations: correspondents argued about the Vietnam War in the Western series Gunhawks, politics in Captain America, feminism in The Cat.

Master of Kung Fu, starring the character Shang-Chi, was originally published by Marvel from 1974 to 1983. Shang-Chi later reappeared in a 2015 mini-series (above) and, earlier this year, headlined his own Marvel movie.

Master of Kung Fu (the 70s series featuring Shang-Chi) published long, thoughtful, pointed critiques of its own content by letter writer William F. Wu, who called out the series for falling back on racist tropes as often as he praised its inventiveness. Wu went on to become a prominent author of science fiction and fantasy.

Letter columns have become rarer in the comics of recent years (not to mention in the magazines and newspapers of today). But a few series, such as The Immortal Hulk and Venom, still publish and respond to reader mail in almost every issue.

Roy Thomas, an early letter writer turned editor in chief of Marvel, following Lee’s departure.

And some of the kids whose responses Stan Lee encouraged so enthusiastically stuck around for the long haul. In 2009’s Amazing Spider-Man No. 600, editor Stephen Wacker asked if anyone whose letters about the series’s first issue, printed in 1963, was still a regular Spider-Man reader—and published replies he’d received a few issues later.

One of them, Fred Bronson, now writes for various TV shows and has published four books about music. “My ambition when I wrote that letter as a 13-year-old was to be a writer,” he noted, “and that’s what I am.”

Douglas Wolk’s All of the Marvels: A Journey to the Ends of the Biggest Story Ever Told will be published on October 12 by Penguin Press