For LiveJournal users, there's The Book You Crew, a community of book lovers, with various entries that are book-related. Booklists, open discussions and the like. Then there's also Book Tards, which I'm asssming would be short for the [quite unflattering] Book-Retards? =P This one has reviews, lists, recommendations, as well as books for sale/trade.
As for Yahoo Groups - the local ones which I know of are Malaysian Book Crossers, part of the BookCrossing.Com network - serendipitous and free/global/exchange of books - its a pretty neat concept - but I haven't tried it myself, too sayang my books to release them into the loving arms of serendipity :P
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The other one would be Book Cafe, a community which I'm part of, founded by Pak Adib. They discuss literature-related stuff, and review/give comments about each other's book list, swap/sell books and the like. I'm not exactly a regular, but I've attended one of their combo meetup with the bookcrossers. I found them a very enthusiastic, jolly and loud bunch of people ;)
Pls feel free to share any other book communities not previously mentioned.
Am also thinking of starting a bookreviewblog forum of sorts. I don't quite like relying on comments box, e-mails, mailing lists, and a seperate wiki to facilitate discussion and the like; perhaps shall use phpBB on the currently inactive BlogMyBook.Org.
What do you think? :)
[Of the idea, not my grammar :P]