Both of Dekay's ideas are good. The space behind the listening area is usually best suited to difusers rather than absorbers, but it depends on how close the listen position is relative to the rear wall. If it's too close, then you might be better off with an absober. That said, the drapes (heavy ones) will work reasonably well as an abosber. Another possibility is to use eggcrate foam (or other absober as Dekay suggests) and cover it by building a frame and stretching whatever fabric (preferably natural and loose woven) over it. As to a diffuser, the books will work very well. If you can't do a book shelf you can also build a diffuser panel, but it's a little more difficult. One way is to use actual egg crates (cardboard type) and coat them with a sealant of some sort (shelac would probably work well). Then you can build a frame and cover it with a suitable fabric (loosely woven, but could be synthetic as long as it is reasonable sound transparent).
