Be careful what you join. It might end up like the comments section here at Bookninja. Except dumber. Considering I can’t wade through the books I  WANT to read, it’s unlikely I’ll ever join a group that tells me WHAT to read.

Today there are perhaps four million to five million book groups in the United States, and the number is thought to be rising, said Ann Kent, the founder of Book Group Expo, an annual gathering of readers and authors.

“I firmly believe there was an uptick in the number of book groups after 9/11, and I’m expecting another increase in these difficult economic times,” she said. “We’re looking to stay connected and to have a form of entertainment that’s affordable, and book groups are an easy avenue for that.”

Most groups are all-female, but there are plenty of all-male and coed ones. Lately there have emerged plenty of online-only book groups too, though — given the difficulty of flinging a drink in the face of a member who suggests reading Trollope — those are clearly a different animal.

And more clubs means more acrimony. Sometimes there is a rambler in the group, whose opinion far outlasts the natural interest of others, or a pedant, who never met a literary reference she did not yearn to sling. The most common cause of dissatisfaction and departures?

“It’s because there’s an ayatollah,” said Esther Bushell, a professional book-group facilitator who leads a dozen suburban New York groups and charges $250 to $300 a member annually for her services. “This person expects to choose all the books and to take over all the discussions. And when I come on board, the ayatollah is threatened and doesn’t say anything.” Like other facilitators, she is hired for the express purpose of bringing long-winded types in line.