This is a book from an author with a Ph.D. in physics, which is fascinating by history and can cite Albert Camus. The book is both about physics and history, much as Steven Jay Gould's Wonderful Life is about biology and history. You can draw many similarities between the two books, not the least the fact that Gould and Buchanan are both good writers.
Buchanan does a good job in transmitting the flavour of theoretical physics in the making. From his book you can learn a lot about the way physicists looks for new problems and try to solve them: making simple (even crude) models of reality, trying to guess the relevant variables, establishing the right connections to experimental data. The critical state is a very suggestive concept, but if you want to do physics you must be very careful to avoid speculation and stick to mathematical generalisation and the effort to make testable predictions.
The main prediction about self-organised criticality is that it is unpredictable. Physicists can point to some very nice regularity in, say, the distribution of earthquakes or wildfires: but nobody can tell where and where the next big earthquake will strike. Why? Because the earth crust is in a critical state, and in such a system "history matters". Every single event in the crust is "froze" in a sequence of events and you should know all of them to tell if a particular site will be struck by a major earthquake or not. The methods of physics show their own limit and give way to historical narrative.
This is really a beautiful book. If you have read some half-mystical book on cosmology or elementary particles, and believe that physics is about metaphysics, get it and come back to quite interesting Earth.