Where to start with room acoustics

Although it's always good to be pro-active, it's possible that you just may be a bit overly concerned with your room at this point. Unless of course it's filled with windows.

Your room demensions should work quite well assuming you place your speakers along one of the short walls.

Unless you're chompin' at the bit to resolve all potential problems beforehand, I would recommend installing your system, dedicated lines, and thick berber carpeting/pad, and then analyze any possible acoustic shortcomings thereafter.

-IMO

You could purchase the Rives Audio test CD and a Radioshack sound pressure meter. The test cd allows you to measure the in room frequency response and will likely confirm what your ears are telling you. From there you can explore solutions.

Have you done a resonant modes analysis of your room, as must be done by Everest in the book you mention? (He does several in Sound Studio Construction on a Budget.) It shouldn't be over your head--just a bunch of divisions to get the modes for each dimension, then listing them in order of size, then looking at how they're spaced. If your 27' were 26', you'd be in some trouble and would need some kind of absorption at certain frequencies. You may anyway, but only a room analysis will tell. Speaker placement is important too, but that can be done by trial and error.

All this applies, roughly, to frequencies 300Hz and below. For above, absorption and/or diffusion will be called for, ideally. Everest should be clear on that. If by any chance you're handy with tools and have a workshop of any kind, I have DIY plans for one- and two-dimensional diffusors of the RPG kind, and for Argent Room Lens clones, which I'd be glad to email to you.

It's not rocket science, as they say, and I'd urge you not to give up too easily if Everest seems confusing at first. Good luck with the project!

The best sounding rooms are the ones that are treated. The LEDE (live end dead end) is a great way to go. Even just doing the wall behind your speakers can help immensely. This is something that can be done without any analysis and will definitely help. From there you can examine room modes, especially in the bass. This is one place it amazes me that so few people concern themselves with. If only they could hear a room treated and untreated. Nobody would leave their room untreated unless forced to for some reason, usually female.

MY experience suggest that you enlist a projessional! You simply won't, that's right won't ever get the best of what you could be getting if you don't get the room right! The room, set up, calibration, tweeking, acoustics, etc, all add up to more than half the sonic equation! If you spend a bit of money having your acoustics all sorted out, considering seating possitions, speaker placements, calibration, acoustical treatments(often not that much, from companies like PMI,etc, which are superb and effective for little!), you'll be WAAAAAAAAAAAAAAAAY AHEAD of where 99.99% of all people mostl ever end up, even with the most expensive gear money can buy! Infact, peoples acoustical spaces are often so poorly adressed and heavily challenged(13x27x8 is a small space acoustically, and needs help and proper considerations adressed indeed), that they haven't a prayer when it comes to getting truely world class sonic playback!
If you consulted many of the acoustical experts out there(again, people like Rives audio anda the people at PMI can be of tremendous and afordable help with simply aiding his help and affordable acoutical treatments, which I had the pleasure to audition at the Show last year).
There's unfortunately no substitute for professional help! These people have spent their lives learnign and perfecting this stuff! The gear is only part of the equation, remember that.
Another "cheep" alternative that can help is to READ READ READ! There are many many acoustical writings in Stereophiles guide to Home Theater mag, HT magazine, AV Interiors mag, etc out there, as well as books like you mentioned. Still, unless you have experience yourself, you will only go so far. Depends on what you want out of it.
To be true however, having sold Hi-end gear for 15 years, at the low to the ultra high end, my experience is that most never get the acoustics issues handled properly, and thus their systems often end up being mediocre to poor overall. That's the way it is.