Note: GBH is a presenting partner of the Boston Book Festival.

The Boston Book Festival returns to Copley Square on Saturday. The 15th annual event is free to the public, with speakers, performances, family activities and more.

Boston Book Festival founder Deborah Porter told Boston Public Radio on Friday that she started the nonprofit organization to fill a gap.

"I was told that Boston was the only major city in the world that didn't have a public event around books," she said. "So I thought, 'Now how hard could that be? I'll just start one.'"

She explained that it wasn't quite that easy. It took a couple years to secure enough funding and arrange venues for the first festival in 2009.

"We started with 80 presenters. We had about 20 volunteers that we scrounged up at the last minute," she said. "And all kinds of people showed up. We had too many festival-goers and not enough seats."

Tomorrow, she said there will be "about 200 presenters."

The Boston Book Festival holds events year-round, bringing people together to celebrate literature, but the eponymous festival is its biggest, attracting upwards of 25,000 people.

Author and staff writer for The New Yorker Adam Gopnik has been invited since 2010. He explained to Boston Public Radio what he believes the "two functions" of the festival are. First is the "avidity of readers to connect with authors." Secondly, it serves as a place for authors to connect with one another.

"You inevitably meet people who you've always admired and never met," he said.

New York Times-bestselling author Chloe Gong, author and Boston College history professor Heather Cox Richardson, and "Percy Jackson & the Olympians" author Rick Riordan are just some of the speakers scheduled at this year's event.

GBH personalities will also moderate eight sessions.

But the event will not just revolve around speakers. Booksellers, publishers, authors and others associated with the written word will have booths at Copley Square. In addition to that, the Berklee Festival Stage will have musical performances starting at 11 a.m.