Is there an introduction to CiviCRM Entity anywhere?
I found the best introduction was this workshop: https://denver2015.civicrm.org/sessions/civicrmdrupal-integration-showcase  The PDF on that page is helpful; the video of the workshop is available here: https://www.youtube.com/watch?v=i2GpnrVGdrc.

Note also that there are significant plans underway for CiviCRM Entity announced this week, see here: https://www.drupal.org/node/2676756