Welcome to the site, Roland!
Most of the books have been discussed and there are "official" threads for them. These threads are not closed and we hope anyone will feel free to add there comments about the books as and when they want to. Don't worry about posting something that someone else has posted. We have a lot of members coming and going and the chances are that any comment you make hasn't been seen by some members. Other threads and topics about books will often be started because a member has thought about something specific that they want to comment on that doesn't necessarily fit with the discussion threads. We are not the sort of site that gets annoyed when members post comments that have been discussed before.
One thing I would stress though is that you think about the fact that some members may not have read all of the books, and therefore don't want to read an important plot item from a book they have still to read. So, for example, if you were writing in a topic about Wee Free Men, it's not a good idea to talk about what happens in I Shall Wear Midnight, as you might spoil the latter book for someone still working their way through the series. If you really must mention another book, you might consider using the "spoiler" tabs.