Reading is a solitary act, an experience of interiority. To read a book is to burst the confines of one’s consciousness and enter another world. What happens when you read a book in the company of others? You enter its world together but see it in your own way; and it’s through sharing those differences of perception that the book group acquires its emotional power.

“There’s a way of interacting through books that you don’t get through any ordinary transaction in life,” suggests Robin Marantz Henig, a journalist who is in three book groups: a women’s group, a couples’ group and a coed and intergenerational group with her daughter, an editor at The New York Times. “It’s like sitting around gossiping about people, only you’re gossiping about characters in fiction, which is more meaningful.”

In book clubs, things can get intense. “We had the most incredible discussion of art, and beauty, and loving something bigger than ourselves,” says Tracy Trivas, whose Los Angeles group often finds itself grappling with “giant issues about the inner life.” When they read aloud a passage from Colum McCann’s novel “Transatlantic” — the scene where Lily, an Irish immigrant, reflects on a painting she’s received from her husband — “half the women had tears in their eyes.”

Ms. Trivas represents a new phenomenon: the professional book group facilitator. A writer with a master’s degree in English literature from Middlebury, she presides over three adult groups, for which she charges up to $300 per session. She also runs a group for children, who nestle under a tree with their parents and read books like “Charlie and the Chocolate Factory.”

“They felt empowered sharing their opinion of the book,” she told me. “I asked them who they would rather have a play date with: Veruca Salt or Augustus Gloop. And if they could make up a different ending.”

We’re also beginning to see authors themselves taking on the role of facilitator. Established writers like Alexandra Styron command $400 to show up and talk about their own books — and that’s after the commission given to Book the Writer, an agency founded recently by the novelist Jean Hanff Korelitz. Authorial proximity has its drawbacks. “No one tells the truth when the author is present,” says a book grouper who has witnessed this phenomenon. “No one is going to insult the author when he’s two feet away from them.”

But there are good things about these home visits, too: They’re a new source of income for writers, and they offer insights into the book that come straight from the source. Book groups are like friendships: Some coalesce and die out in a few years, others last a lifetime. Susan Shapiro, an artist whose group has just celebrated its 40th anniversary, recalls that it started with “young moms in the park who wanted to have more to talk about than kids in the sandbox.”