Amatai Etzioni's The Spirit of Community is intended as a communitarian text, outlining the responsibilities of each individual in a community to the whole so that all members prosper. This is similar, although explicitly different in fundamental principles, to Utopian socialism.
In the introduction, Etzioni writes:
Communitarians are dedicated to working with out fellow citizens to bring about the changes in values, habits, and public policies that will allow us to do for society what the environmental movement seeks to do for nature: to safeguard and enhance our future.
(Etzioni, The Spirit of Community, Google Books)
Although he is clearly not an advocate of right-wing politics throughout the book, many of his stated goals are similar: establishment of personal responsibility, for example. His audience should therefore be individuals who are unsure of their political bias, or concerned with the state of the country over their own personal issues. Etzioni also seems somewhat uncertain where Communitarianism falls on the American political scale: he oddly lumps libertarians in with the ACLU. Nevertheless, his book is best read by left- or center-left leaning people, as they are most likely to agree with his ideas.
Your question made me smile quite a bit, actually, because I am going to be a bit politically incorrect when I say that Etzioni's The Spirit of Community is meant to be read by people who may generally be unsure of their exact politics, but who are liberals or people leaning to the left of the political spectrum.
Further, though, let's look at a few specifics as to why this is the intended audience. It is, in its purest form, a list of responsibilities for each member (or each "individual") as a member of a utopian, socialist community. These responsibilities are designed to help the community prosper and further its label as a "utopia." In two adjectives, Etzioni describes his goal as to "safeguard" and "enhance" the future by calling upon the responsibility of these individuals. Further in Etzioni's The Spirit of Community, he calls these individuals by a particular name: "communitarians."
You can see how Etzioni's The Spirit of Community, then, fits within Amitai Etzioni's biography as someone who considers himself both an Israeli and an American and is definitely a sociologist. It is the "communitarianism" that he espouses in The Spirit of Community that is his lasting legacy. Etzioni is the leader of the Communitarian Network, which considers itself non-partisan and has a goal of bolstering the foundations of society that are social, political, AND moral.