Share 'Author V. Author'

I'm sure this has been mentioned somewhere, sometime before -- but it's a question that persistently baffles me. Sharing. We all read books. Most of us are writers. We encounter other writers, though this site, GoodReads, Facebook, Twitter and many other sites. I'm curious about the enigma of reciprocity. I have helped promote other author's books on many online sites. I do not find they reciproc…