A big book on the big topic of 'how do we all get along' by one of the biggest of living philosophers.
I'll not address the details of the argument or Habermas's place in left-wing politics. Instead, I'll address the intellectual and cultural context.
What Habermas says he is doing is looking for a way to hold societies together that are no longer composed only of one ethnic group; that are no longer made up of adherents of one religion; and are no longer made up of people who accept one myth of their nation or one philosophy of life. We wouldn't need his contribution here, he is saying, if we were not in "postmetaphysical" times--by which he means two things. First, he means that we're in a scientific, secular era when the educated classes, anyway, of major Western countries can no longer be convinced of much of anything by *religious* arguments. Religion doesn't command much belief among social elites, and many others, let alone the kind of universal belief it once inspired. And theology has long since been driven from the position of being 'queen of the sciences' by physics. The second thing he means by "postmetaphysical" (which he uses instead of "postmodern") is that we live in a time when it's hard for any of us to believe that only what we believe is true, and that what we believe is totally true...because our world is so interconnected and we are aware of so many different religions and worldviews people have. That is, religious and worldview pluralism relativizes the authority any one religion or worldview could have now.
Mostly Habermas thinks our "enlightened" state of cosmopolitan equality is really good. But he acknowledges that we've lost something in losing the certainties and meaning and ethics of religions. Among other things we've lost is the social glue that holds citizens of countries together. Since Habermas is a social philosopher of hope, who wants to prevent a Nazi regime and a Holocaust from ever happening again, this is really important to him.
So after saying why socialist welfare states, with their paternalistic governments, and unregulated capitalism, with its discrimination against those who are such losers as to not be affluent, can't be the way forward, he then surveys and rejects other options. Of course, the way forward is his theory, which in his lingo is a constitutional deliberative democracy with a free public sphere and a vibrant lifeworld. Never mind all that, unless you want to get into his theory. The force of it here is that, in a way most people afraid of getting speeding tickets would not expect, he, as a leftie, sees The Law as the best means for keeping all of us together. Even if we don't respect each other so much, basically, if we respect the law we can get along. Even if we don't care about each other so much, if we do as much for each other as the law demands, society will be livable. So the right kind of law makes possible a peaceful society of people who radically disagree on really basic stuff that would often make people violent.
The book is designed to sort out the right kind of law. It is the kind that you can obey not just because you'll get in trouble if you don't, but also because you can agree in principle with how the law was made (even if you don't like the law itself). And the right way to make laws is for people to talk long enough and openly enough with each other in political publics and fora to come up with basic rules of the game we can all live with.
Highly technical, highly abstract, assumes you know basic stuff about Aristotle and Kant without him explaining it, amazingly comprehensive. Underrated in the US because it's not done in the usual Anglo-American way, but not only great for legal theory types, but also for people doing Rawls or Rorty or Derrida or MacIntyre. And for systematic thinker people, think of Between Facts and Norms as Habermas's equivalent of Aristotle's Politics or Hegel's Philosophy of Right. If you like the Olympic pool these guys swim in, this is gold medal contender material.