Links and resources
Books
The Master Handbook of Acoustics By Frederick Alton Everest, Google books
Audio engineer's reference book By Michael Talbot-Smith, Google books
Sound engineer's pocket book By Michael Talbot-Smith, Google books
Educational
Acoustics and Vibration Animations - Dan Russell, Kettering University
Studiotips.com, some tips... and a Forum about acoustics for small rooms.
Material (commercial)
Real Traps, Absorption pannels and good "How To" section, videos and more.
Ready Acoustics, Simple design with good material, full range of absorbers, even Do-It-Yourself kits.