The functioning principles of Agile (and process improvement more broadly) can be found in lean manufacturing and six sigma. These concepts date back to just after World War II and include error proofing, eliminating waste, creating flow, adding customer value, and empowering workers. If you are interested in deeper understanding I suggest reading the above links and:
- 14 principles of the Toyota Way
- 2 pillars of the Toyota Production System (Just-in-time and smart automation)
- 5S methodology
- Total Quality Management
- Deming’s 14 points
While more comprehensive, notice how similar these ideas are to the often referenced 7 principles of lean software development. One of the items I wanted to highlight when editing the Agile definition on Wikipedia was that the principles predate the Agile Manifesto in 2001. I think the idea that these concepts were crafted there for the first time sometimes stifles creativity in how Agile gets applied and creates the mind-set that practitioners need to invent as we go.
Going back to established principles and their associated tools and practices offers a proven base to grow from and an opportunity for surfacing new ideas for continuous improvement. I also find in them core ideas for applying concepts outside of development teams to grow a more agile enterprise.
Oh, and of course, I recommend my own Laws of Agile Development series. Please let me know if you are aware of any others I’ve missed.