There might be some similarities in book clubs, especially if you break them down by different criteria, but i wouldn't say there's one defining type of bookclub. The one that frequents the coffeeshop where i work is comprised of middle-aged women (a couple look pretty good
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), and their bookclub is really an excuse to gossip about everyone in town (myself included). They've been reading "The Purpose Driven Life" for six months now. i've seen other groups that are more structured and actually discuss the book, with references to other works, seldom delving into other discussions. i think what sets the clubs apart are the ones that have a moderator or leader that controls what goes on in the group. Without that person, the conversations break away from the book(s) at hand, or even literature in general.
i'd join a bookclub, but the fact is that most of them read fiction works, and i'm pretty much strictly a non-fiction guy. i could see myself joining a special interest group (non-anime or gaming) though. Depending on the group, it would be a great person to meet someone, because you obviously have a common interest.