Bookshelves & Rooms

I'm moving to a new house and a new stereo room (15' x 23'). I would like to line the room with bookshelves to hold my thousands of records. How will my system "see" the bookshelves? Will the room size be "reduced"? Do I treat the bookshelf fronts as the room sides for placing the speakers?
Does 15x23 become 13x21 if I have bookshelves 12" deep?
I know there is a sonic difference between bookshelves with and without backs. What if I only had bookshelves on the non-speaker half of the room? All suggestions appreciated. Thanks. Steve
The best place to start is the rear wall behind your listening position (diffusion and bass absorption).

Sharp cabinet edges should be avoided within about 3 feet from the front baffle of your speakers (edge diffraction). I would keep the side walls clear if you can so as not to "crowd" the speakers...leave 3 or 4 feet between speaker and side wall so you can sit further back in your room and still get good imaging. Acoustic panels/fabric wall hangings on side walls at "mirror" points can always be added to your room if it sounds too "live" for your tastes....however bookshelves are a more permanent addition that you can't adjust easily to gauge their impact.
I didn't realize how much my room echoed until I removed my wall mounted bookshelves for repainting. They're all on the side walls closer to the back wall than the front where the components and speakers are.
Shardorne: It sounds like you're talking about the dead/live room concept, with bookshelves behind me and barish walls behind speakers. I like that idea, but have big LP storage problems. The more of the room I can use for bookshelves (with or without backs?), the better. I'm trying to get a feel for how much of the "hard" room dimensions I lose when a bookshelf or cabinet is on the wall.
Gunbei: Sounds like your setup is like Shardorne was recommending, bookshelves behind the listening position. What do you mean by wall mounted? Did the bookshelves have backs? What did you have on the bookshelves (LPs, I hope). Thanks.
Hi Suttlaw,

I have some CD wall cabinets I purchased and also some I constructed to hold books, DVDs, etc. They're basically long horizontal wood cabinets approximately 90" long, 18" tall, 11" deep with open fronts. They have no sliding doors in front and the structures are bolted to the wall studs and mounted above my desks. LPs, CDs, DVDs, might be useful in diffusing some of the sound, but I have a feeling that the irregular nature of what's in my "book cabinets" is more effective at reducing side reflections.