Albert Helfrick’s works are copyrighted. Distributing or downloading a free PDF of the full textbook without the publisher's permission is a violation of copyright law. While older editions may occasionally be found in libraries, the latest editions (which cover modern tech like ADS-B and NextGen) are almost always strictly protected.
Dr. Albert Helfrick is a professor at Embry-Riddle Aeronautical University and a former designer of avionics test equipment. His book is a standard reference because it covers: Core Systems