Superheroes are good business these days.

Marvel's latest superhero movie "The Avengers" is dominating the box office, netting more than a billion dollars already.

And off the screen, capes, spandex, and muscles are also selling pretty well on paper, in the comic book industry.

"In five years we've doubled our business,” said Jay Long, owner of Heroes and Dreams, a comic book shop just outside Jackson, MS. “We've grown consistently by 15-20 percent every year, sometimes a little more than that."

Comic book sales topped out at more than $660 million last year by some estimates. But what makes comics so unique is how they’ve survived and even thrived during a digital revolution that’s seen the downfall of other paper-based media.

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"Where other print formats are kind of fading or diminishing in sales, comic books are on the rise, not only in the digital format, but also in the paper format," Long said.

While the digital market for comics is still relatively new, it has exploded in the last year.

“We’re seeing an increase (in demand) across the board,” said David Steinberger, CEO of Comixology, the industry’s largest platform for digital comics.

In the first quarter of 2012, ComiXology had revenue 450 percent higher than the first quarter of 2011. And between January and April of this year, digital comic subscribers downloaded more than 15 million copies.

During that same span, paperback comic sales also grew 20 percent from 2011, having sold more than 24 million copies through April.

To put that growth on display, the industry just held its largest ever annual Free Comic Book Day. More than a million people in 50 countries visited their local comic shop to get their hands on some of the 3.5-million books given away.

“It is Christmas for Geeks,” Long said with a laugh.

The allure of free comics helps bring in new fans and customers to local comic shops, and capitalizes on the huge mainstream popularity superheroes have achieved in Hollywood.

Zehari Nickelson is one of those new and excited comic books fans. She and four friends made their first visit to Jay Long’s comic shop, Heroes and Dreams, after seeing "The Avengers" in theaters.

“I’ve never really been super into comics per se,” Nickelson said. “But I want to get into comics, because…I like Iron Man.”

Long says his store sees more and more new faces every year. And Steinberger says with Hollywood pushing superheroes into the mainstream, he expects the digital market to keep growing as well.

“We’re still a very small percentage of the print market in terms of size," Steinberger said. "But as I’ve said I think there’s an opportunity to grow the market pretty tremendously."