Professor Galbraith's central thesis is that the amount of wage inequality (the difference between what the wealthiest make and what the poorest make) is tied directly to a number of key factors: the unemployment rate, the interest rate, the strength of the dollar, and so on. Essentially, the lower the unemployment rate, the less the degree of wage inequality. I am simplifying his thesis considerably here, but this is the essence of it. There is a considerable amount of technical research presented that supports the thesis and demonstrating a strong correlation between wage inequality and unemployment.
So, Professor Galbraith makes a persuasive case that the government can affect wage inequality by making certain policy decisions, such as lowering unemployment, raising the minimum wage, weakening the dollar against international currencies, and so on.
But, what is missing from the book is a serious justification of why the free market should not dictate the value of labor. In other words, from a normative standpoint, why should workers be paid two or three or more times the value of their labor? There are egalitarian arguments to be made, of course, but at the same time, if the value of Bill Gates' labor is, say, 100 times more valuable than the work of a custodian, why should Gates' wages be limited to, say, 30 times that of custodian? (At one point, Professor Galbraith suggests a 30-1 limitation.)
In the end, this is an impressive and valuable work. You may or may not agree with the policy decisions that Professor Galbraith supports, but his analysis must be reckoned with.