F. Alton Everest, "Sound Studio Construction on a Budget", has a lot on isolation, though most of it may be advice for building, not for modifying. But it's a great reference for all kinds of acoustic treatment. I'd give it a try. Maybe your local library can get it for you, interlibrary loan.
Sound Deadening
I live in a condo that has less than satisfactory sound insulation between me and my neighbor. I rarely hear my neighbor, but I suspect that they hear me quite often =] . I own the place, so I am not opposed to doing some major modifications (hopefully inexpensive). I found some good stuff at: http://www.owenscorning.com/around/sound/products/index2.asp but I am wondering if anyone else has any other information. Thanks!!!!