Model Helicopter Technique ("the color is in the content" - a great publication for those serious about RC helicopters)
MotorBooks.com (check out the Zenith Books sub-site for many aviation-related books including Martin Simons' 3rd edition of Model Aircraft Aerodynamics)
QuietFlyer Formally (Sailplane and Electric Modeler) or (S&E Modeler)
Traplet Publications (publisher of numerous modeling books and videos, including the Harry Higley series, Don Edberg radio programming and other titles)