Try goodreads. There are loads of groups on there for different tastes. The Book Vipers is the one I'm in, and most of the members are on mn too I think, I definitely joined because of a recommendation on here.
Mumsnet Book club! Make sure you're signed up to receive the book club newsletter which has info about all the books we're reading (also check out book of the month discussion topic). There's also weekly book giveaways where we ask Mumsnetters to read books and discuss them on the discussion thread or review books. (Check out the book giveaways discussion topic).