This book begins with Thomas Hobbes thesis that a working modern society requires a strong state. But, of course, having a despotic stat does not really solve the problem. Liberty cannot flourish in either a state of anarchy (Absent Leviathan) or a tyranny (Despotic Leviathan). What is required is a state whose powers are controlled by a strong civil society with democratic traditions, a Shackled Leviathan.
The Narrow Corridor of the title refers to the thin area in a diagram where the x- and y-axis denote the power of the state, and society, respectively. The part of the diagram where the state has overwhelming power is the Despotic Leviathan, where the state is weak is the Absent Leviathan, which often means a clan-based society.
I find the analysis convincing, in general. It is not so much a theory of society as a way to analyse what happened in particular cases. The requirements for a society to get into, and stay inside, the Narrow Corridor are always dependent on the particular historical circumstances. The authors tell stories about what happened in a number of cases, and although I cannot claim to be an expert, I cannot spot any direct and obvious misreadings of history.
The main take-home message I got from this book is the contingent nature of a state of liberty. Often, the liberal view seems to be that democracy and a free society are somehow natural, and other types of states and society are perversions. In contrast the view in this book is rather that a free, individualistic society is very much a special case, which requires hard work to reach and maintain. I believe this is correct.