I am a libertarian, and I love this book. With enemies like Cohen, libertarians don't need friends! Cohen's arguments here are two-fold: First, he wants to argue against libertarianism, or, more specifically, Nozick; secondly, he wants to justify doing so to his liberal and left wing philosophical colleagues. This requires his showing why libertarianism needs to be taken seriously. As an exempler of this dichotomy, there are Cohen's two chapters on a basic premise of libertarian arguments, self-ownership. The first chapter is made up of several arguments as to why the notion of self-ownership is appealing, coherent, and has content; it not only defends self-ownership as being coherent against Kant, and possessing content against Dworkin, but also the thesis that, if people own themselves, then they cannot be forced to help others, and forcing them, via taxation, to help others as a condition of helping themselves undermines that self-ownership.
But the next chapter turns away from criticising the left to criticising libertarian arguments. Cohen argues that libertarians are wrong to argue that taxation is slavery, or that arguments used to support redistributive taxation undermines self-ownership. He also argues that Nozick's appeal to Kantianism can't support the thesis of self-ownership, and neither can pleas for personal autonomy. He notes that only one argument from Nozick can go through, and that is that taxation undermines self-ownership, as explained above. But, Cohen says, Nozick can't really support that position, since he favours courts and police in his minimal state.
And that is one of the benefits of Cohen to libertarians - many of such counter arguments are very weak: Nozick did support a minimal state providing courts and police and national defense. But nowhere in his libertarian workd did he defend taxation to fund that minimal state, and even if he did, many libertarians don't - Cohen's argument here shows that commitment to self-ownership should commit you to radical libertarianism, ie, voluntary statism, or market anarchism.
In all, Cohen is essential reading for any student of libertarianism, either for or against it, since he provides great and tough arguments against libertarianism for opponents of it, whilst also providing tools and arguments for libertarians to refine their own arguments and their own positions against their opponents.