You can get kits to seal the doors for about $150 a pop. You treat the rooms "professionally" with Owens corning 703 fiberglass insulation panels that you wrap up in fabric. The standard size is 4'x2', but really you can make and or order them in any arrangement. You want these to ideally be 4" off the walls. The exact distance off the walls and thickness of the insulation determine what frequencies are absorbed.
Here are some basic tips.
You can use heavy theatre curtains for the glass door (probably the cheapest option). You want to start with bass traps 4" thick panels.Put these in every corner up to the ceiling. You also put one panel in the right angle between the wall you face when you mix and the ceiling. Behind you you ideally want a diffuser. A bookshelf or even the card catalog you mentioned works well. Diffursers are the most expensive, overpriced, and difficult to make things. In front of you at mix position you want one 4x2x2" think panel behind each monitor. You then want a few panels along the left and right side walls. How many depends on what issues you have with the room, and how dry you want it to be. Last would be to hang some 1" panels off 6" off the ceiling, covering about half the surface area ceiling. All the distances off the walls for everything are ideal. In the end you do the most you can given the room. The space is important as the sound punces off the walls and gets absorbed twice.
This is a general setup I would advise. I built panels for this setup for about $500. I made wooden frames, bought the fiberglass (you will most likely have to do that at a special insulation retailer), and wrapped it in 100% polyester cloth. The polyester is more fireproof than cotton as it just melts instead of catching fire. You can buy specific cloth made for sound transparency and that are fire rated but it will nearly double the cost of making the panels.
You can find videos about hot to build these panels on youtube.
I honestly never want to work with the fiberglass again. There's a company here in Seattle that makes insulation for room treatment out of recycled denim. Rockwool is also an option. I would use that. And instead of making would frames, I would use those long right angle cardboard pieces you get in packages. And wrap everything in polyester just the same as before. I haven't yet tried this method, but I think it would be so much easier.
In the end you really want to treat problems in your specific rooms. If anything do the bass traps and panels behind your monitors. Unless you are in a room of at lease 30' you have bass problems because the bass waveforms never finish their cycles before hitting a wall. This is assuming you are going to be recording and mixing in you space. For a jam space u can get away with less. In my jamspace right now, there is just a 1" thick piece of rockwool about 4' tall wrapped around the entirety of the room and 4 panels up higher one on each wall. That will make the room less live. But it doesn't really treat the room.
To fully tread the room, you use the room dimensions to find you standing waves and go from there. You then adjust where the panels are and how far they are off the wall to fix certain frequencies. then you get a measurement mic and some software. You then blast the room with pink noise and sinewave sweeps and get your frequency response. getting a good mic and the measuremt sofware sets you back at least a few hundred on the cheap side.
There are also the monitor controllers by JBL and stuff that come with mics and some software, it then EQ's the speakers to fix the problem with you room. A lot of studios use a physical treatment setup in conjunction with this, for a mixing room. A live room (the recording room) just has to be treated properly. If you have the two rooms in one, just treat it physically.
If you buy door kits, panels, curtains, etc... You can treat the room for <$1000. IMO better than any piece of gear or software you could buy for mixing/recording at that price. I'd rather have a treated room with an SM57 recording to a portastudio. .
This book helped me a lot: http://www.amazon.com/Build-Budget-Recording-Studio-Scratch/dp/0071782710
It goes more in depth about soundproofing as well which is a completely different beast. I would recommend starting with high density vinyl sheets and door kits for cheaper soundproofing options.
Owens corning has some great info about acoustic treatment on their site.
Here's some good instructions I was going to use to build quality diffusers out of bamboo. I ended up moving though.