Over the last couple of years, I have read many books and sat through many videos on  Integral Theory, Spiral Dynamics, and Metamodernism so that you don't have to.

Below are my best recommendations for further investigation, with some notes and cautions.

Note:  Many of the resources below focus only on Spiral Dynamics. Spiral Dynamics is an (over)simplified version of Integral Theory that focuses on practical, real-world applications of the ideas and a general avoidance of any metaphysics (things that are outside of science, reason, and observation).  Integral Theory is a much more spiritual and metaphysical view of the same phenomenon.  Metamodernism believes that Spiral Dynamics is too simplistic and Integral Theory is too spiritual and attempts a more scientific, rigorous theory.  But they all refer to the same main subject: adult development theory.

If you don't have very much time, watch the video introduction. If you only have time to read one book, read A Brief History of Everything, by Ken Wilber

My written introductions (each 15mins read):

• Integral theory part 1, and introduction to the evolution of human consciousness
• Integral theory part 2, a diagnosis of our political world
• Integral Theory, MHC, and Metamodernism...how they all fit together

Video introductions of Integral stages, states, quadrants:

https://www.practicalintegral.com/resources

Books

• Ken Wilber - Ken Wilber is known as the most prominent Integral philosopher in the world.  As a first exposure to Integral, one has to be aware that Ken has no problem talking about spirituality, Freud and Jung, and general ideas outside of science and evidence.  If this is something that might be a problem for you, you should start elsewhere, perhaps with Spiral Dynamics.

  • Trump and a Post-Truth World - Very short (161 pages) and very practical introduction to integral thinking using the real world example of the Trump election in 2015.

  • A Theory of Everything - This is a relatively short book (196 pages) that will give you a somewhat dated but comprehensive summary of Ken Wilber's Integral Philosophy.
  • A Brief History of Everything - Ironically, not brief. More dense than "A Theory of Everything".  A good thorough summary of all of his ideas, but also a bit dated.  It's also written with a bit of a philosopher's voice, so if you are not familiar with other philosophers you may have trouble keeping up.  The 20th anniversary edition has an interview at the end that brings the book up to the current state of the theory.

  • The Religion of Tomorrow: A Vision for the Future of the Great Traditions - Perhaps the greatest, most detailed, and most comprehensive integral book ever written.  This book is not for the faint of heart, nor those looking for a casual introduction. It is very long, but wastes no space and is packed with epiphanies.  

• Hanzi Frienacht - For if you don't like Ken Wilber....  Hanzi (pseudonym) is relatively new on the scene and has somewhat of a different take on the whole Integral/Spiral Dynamics field.  He calls himself a Metamodernist, meaning post-postmodernism, and his theory is that a version of Integral Theory is breaking out in the real world in the Nordic countries.

  • The Listening Society: A Metamodern Guide to Politics, Book One - A walk-through of the political environment in the Nordic countries and how these countries are all moving toward an inevitable post-postmodern government based on Metamodern ideas.  Also lays out a version of adult development stages that relies on a very well supported scientific field called the Model of Hierarchical Complexity.   This is the most detailed discussion from this angle, though incomplete.

• Steve McIntosh - Steve is an integral philosopher who came in as a second wave behind Ken Wilber.  Steve is a slight critic of Ken, in that he thinks Ken is a little bit too comfortable in the spiritual realm.  Steve is a better bet for people who want to be talked to like a philosopher.

  • Integral Consciousness and the Future of Evolution - A traditionally philosophical introduction to integral, focusing on the genealogy of the ideas and how the ideas are backed up by other modern research and philosophy.

• Don Beck - Don Beck is the creator of Spiral Dynamics, when he came across the work of Clare Graves back in the 1980s.  Spiral dynamics is a simplified fork of integral philosophy, and focuses on practical, real-world, nonmetaphysical applications. He turned Spiral Dynamics into a consulting firm for companies.

  • Spiral Dynamics - This is an old book and needs updating. Also not available on Kindle. It is still the most straightforward and readable of the introductions. I recommend this to my libertarian and conservative friends as a gentle introduction.