The Maryland Open Source Textbook (M.O.S.T.) Initiative is a huge source of support for open educational resources, open access, and open pedagogy in Maryland. Find OER through the M.O.S.T. Commons or Stevenson's M.O.S.T. Commons Hub.
Three states -- Maryland, California, and Georgia -- maintain large statewide OER databases. Try these quick searches that are already set for your discipline:
These sites offer multiple books:
Here are some individual books on the topic of communications: