COMSAT Users Guide : For Maritime Satellite Services Through the U.S. Coast Earth Stations by 
Communications at Sea by
will help you cut through the sales hype and see what systems really work for the cruising sailor.
GMDSS handbook : handbook on the global maritime distress and safety system by 
International Code of Signals by
The International Code of Signals is an international system of signals and codes for use by vessels to communicate important messages regarding safety of navigation and related matters. The code covers visual, sound, and radio communications.