I'm a member of two book related Yahoo! Groups. I'm not a very active member, but I do try to read as many books as possible from their schedules, and I try to influence their schedule by nominating books and voting in the polls. I follow the debates about the books that I've read myself, and sometimes I join in.
First there is BookiesToo with about 1000 members, and a schedule of two books per month. And the other is Bookworms_Reading_Group with about 3000 members, which reads one book a month.