I have 27 kids and they are sitting in groups of 6 or so with desks faced together as "table groups". I like the kids to sit together because we do a lot of group/partner work. They do tend to be chatty in this arrangement of course. With my class though, they may be chatty no matter how they sit!
I have a "meeting area" that also serves as a reading corner with bookshelves on 2 sides, several adirondack style plastic chairs and a small sofa. The kids have assigned jobs each week and that determines who gets to sit where to avoid any rushing to get a certain spot. We do morning meetings, class meetings and gather here any time I want them close together to hear/see something important up close.
My classroom library is pretty large and is organized into separate book baskets which are labeled with laminated genre titles (e.g. Realistic Fiction) that are attached with metal binder rings. I have done this for many years and I always love it! The kids can see the books so much easier and can find the ones they want much better this way too.
I don't do centers like I did teaching K or 1 but it sounds like fun. I do try to have lots of things for the kids to get invloved in if their work is done. Reading is always a choice as well as writing (we do Writer's Notebooks and some kids love to write). Also I do a Brain Challenge program similar to Heather Renz (awesome website if you haven't seen it). We do ongoing art projects too like weaving.
Hope this helps!