Project Retrospectives - A Handbook for Team Reviews by Norm Kerth
Sort of the “Godfather” of the modern day, agile retrospective is Norm Kerth. I always try and mention norm and his work as a means of giving folks a sense of the pre-Agile legacy of retrospectives. Point being, it pre-dates agile approaches.
The other nice thing about Norm’s work is his notion of “safety” in retrospectives and his Prime Directive. I almost always reference the prime directive at one point or another with each of my clients and in my teaching. It epitomizes the “mindset” of a healthy retrospective.