"do it yourself" acoustics

Full bookshelves on side walls - both, to get rid of the squareness or your room?

Agree with Ozfly that active room correction is the way to go. Suggest you also look at TacT's room correction units.
FYI, they recently lowered their pricing via direct sales.

That speaker and your dimensions are the poster child for our PARC. I don't normally post something that strong about our products--but in this case it's just the truth. Ozfly already mentioned it. In the other areas--absorption and diffusion, be sure to check out our resource section--there's lots of free advice and articles there that may help you.

Rives Audio

I am in a similar situation. I have 12 by 12 room and the squareness(is this a word?) is the killer here. I've had some success in making it work.

1) If this is a dedicated listening room and you can do it, position the speakers at 90 degrees (in other words make a diamond shape rather than a square) this is the best thing you can do to improve sound in your square room.

2) if you have bass issues, bass traps DIY or pay a pile of money for commercial ones. My bass it solid, not over powering so I removed my DIY traps which worked too good.

3) Flutter echo

4) 1st reflection points, don't forget the 1st reflection points on the ceiling and back wall.

Flutter echo made my room very bright, also I put acoustic foam in the corners.

Hope that helps. My room is tolerable.

Rob

In support of your posted premise "do it yourself," may I humbly suggest you delve into one of the Kings of DIY's website "www.geocities.com/jonrisch" . You will find a plethora of diy acoustic projects to occupy yourself. I built his bass trap panels and found them most excellent. While they will not have as great an effect as pricier equalization solutions, they are much better than simple foam, quilts, etc. The foam & fabric route is only effective in the higher frequencies, whereas the bass traps affect much lower frequencies as well. How low depends on how thick you construct them.