William J. Maxwell, a professor of English and of African-American Studies at Washington University in St. Louis, coedited “Romance in Marseille,” which Penguin Classics published. He said Ms. Rotor has a more “post-Eurocentric sense of what a classic can be,” and has “consciously tried to widen that definition.” While running the imprint, Ms. Rotor has broadened the number of its books by Asian-American, Caribbean and African writers. She wants to work on adding more Latinx and Native American writers, as well. Ms. Rotor’s expansive definition of classics applies to genre, too — she has brought more horror, science fiction and fantasy under the Penguin Classics mantle.

In the past, the inclusion of these books in discussions about classics, or their use in college classes, was a matter of sometimes heated debate, but Mr. Maxwell has seen those arguments quieting down in recent years. “We now understand that there are multiple canons, that they’ve shifted over time,” he said. “Canons don’t come down from God. They’re shaped by sociology.”

The books being added to lists of classics, Mr. Maxwell said, are broadening and complicating our understanding of history. “Writers themselves haven’t respected the narrowing of the canon in many cases,” he said, citing as an example how scholars now have a deeper understanding of the Harlem Renaissance as a more geographically and culturally expansive period than was once believed. “Canons are not just about finding new stuff,” he said. “They’re about finding space for old stuff that didn’t seem economically and culturally commodifiable.” Ms. Rotor agrees, though she also revels in unearthing new work. “There’s just so much more crate-digging,” she said. “It’s endless. You never know what you can find.”

Though there is growing consensus across the book industry about the need to diversify its offerings, including titles from its backlists, there have been moments of controversy about the best way to go about it. In February, for instance, Barnes & Noble came under fire for promoting classic books with new covers portraying characters whose race hadn’t been specified, but were long presumed to be white, as people of color. The initiative was canceled after a barrage of criticism from those who thought promoting books by black writers during Black History Month would have been just one of many better, more appropriate possible strategies.

Still, Mr. Maxwell notes the significance of publishing under the Penguin Classics banner, with the books’ instantly recognizable black spines and penguin logo. “You can sort of think about these books as one unit, and you can start to see the cross pressures in the way that they’re put together,” he said.

A significant portion of Penguin Classics sales are from course adoptions by high schools or colleges, where students increasingly expect a more inclusive selection of texts. Ms. Rotor said her team focuses on giving readers opportunities to “intellectually and culturally gather around a book,” often in a classroom.

Ms. Rotor said she and her team are “listening to communities” when it comes to deciding which books to tackle next. “It’s nothing to do with our editors saying, ‘We have just deemed this a canonical text,’” she said. Instead: “We’re seeing what people are expecting from us, and we want to bring those stories, and a more diverse and inclusive program of stories, into our series.”