Room acoustics basics

Check out some Ethan Winer YouTube videos.

agree with @willy-t  on Jim Smith's "Get Better Sound".  Much of his very understandable non-STEMmy book focuses on room acoustics and how the room is the #1 component in your system.  Worth the purchase--helped me a great deal.

I used the book Master Handbook of Acoustics. Loads of great information.

After I purchased speaker cables from Morrow Audio I received an offer to sign up for their Tweaks Newsletter. I received the first one and it was pretty basic ( what the OP wanted ) yet informative. It quickly and informatively described treating the wall behind the speakers and the wall behind the listener with easy to obtain and affordable materials. You don't have to make a purchase from them to subscribe to the newsletter. Just go to:

Jim Smith wrote “Get Better Sound”

which includes optimal speaker placement and room acoustics basics

 Check it out 

Good luck Willy-T

I called GIK Acoustics for help but  they are selling their products 

Couldn't believe how much it help the sound from a big ball of sound to an amazing soundstage with such focus and bass control 

you can check out my room pics in systems

https://digistar.cl/Forum/viewtopic.php?t=557

I found this to be a very interesting read.  It touches on many of the accepted theories of room design and adds some new thoughts.  It includes a room dimension calculator so you can plug in different numbers and see how they change the room.  I like comparing the same numbers on the Bonello formula to see how close the two charts can get.

Here's a relatively recent (2014) book on acoustics that might be valuable.

Acoustics of Small Rooms

Odeon Room Acoustics Software. Their site is very informative. 

Earl Geddes wrote what might be considered by many a definitive book on the subject of acoustics.