• Microsoft Flight Simulator 2004: A Century of Flight:
  Official Strategies & Secrets
• Aeronautical Information Manual (FAR/AIM)
• Say Again, Please: Guide to Radio Communications
• Pilot's Radio Communications Handbook
• The Pilot's Reference to ATC Procedures and Phraseology
• Aircraft Owners and Pilots Association (AOPA)
• Official FAA Charts

These recommendations are taken from the FS2002 "ATC Handbook",
page 10 and apply equally well to FS2004.