A disclaimer: I only read the first chapter, the conclusion, the postscript, and the appendix of this book. These are the parts that focus on the theory itself in idealized and theoretical terms. The rest of the book, chapters 2-6, I chose to skip due to time constraints, and consists of historical applications of the theory. For someone interested in the history of the New Deal, the 1988 and 1992 elections (the latter being apparently of interest to modern politics, since the 2008 election was supposedly a rehearsal of it), these chapters would certainly be worth reading.
Ferguson's theory makes everything make sense, both historically and politically. After years of being inundated with the notion that voters have been controlling US politics for the entirety of our history but feeling that this understanding was false and hollow, the investment theory of elections brings a refreshing clarity and focus to the area. Historically, we now know where to look to find the driving forces in political events. It resolves what for me was a lingering doubt about the capacity of ideas and ideologies to independently impel economic, concrete forces in history. My understanding of this relationship is now considerably clearer: the ideology of the populace is not dependent on economic forces, as I had once thought; the reason it is not the driving force in supposedly democratic elections is merely because economic inequality prevents the people from having a real influence on those elections.
This is the political implication of the book, which Ferguson could have made more of. The book is clearly meant for an audience of political scientists, and I think a popularization of his conclusions is called for (and may already exist?) Anyway, what Ferguson wants us to understand is that big business controls politics; that this is most emphatically NOT a democracy. The 'golden' rule the country, as it were (and of course this doesn't only apply in the US).
In order for democracy to exist, real, deep election reform is necessary, reform that heavily subsidizes politics in a way that makes campaigning available to everyone, not just those willing to carry out the policies of rich investors. Until this reform takes place, the interests of our government will be entirely at odds with those of most voters, and thus the policies it enacts will be, to varying degrees, harmful to those voters. The examples of this are almost limitless, but see for example the government's position on climate change, food reform, healthcare, foreign policy (particularly the defense budget itself), pollution regulation, etc.