There is the
book club forum. Every month a book is chosen to be read and discussed and as far as I know, the discussions are never closed. You can even suggest and participate in the selection of the monthly book.
Otherwise, I think most informal book discussions take place in the
Reading Recommendations forum. The trick is to find a book that many people have read and can discuss.
The book club discussions used to be in that forum too; it is very recently that it branched off.
If you want to include spoilers, just say so in the thread title and people that don't want to be spoiled will know to stay out.
