Miller has written a good book that covers a lot of the territory in contemporary meta-ethics. The book is structured around two basic debates within the field: the debate between cognitivists and noncognitivists, and the debate between realists and anti-realists.
The presentation of the material reflects the canned history of twentieth-century meta-ethics that should be familiar to anyone with some knowledge of the area. Our story begins with G. E. Moore's Principia Ethica, which is the subject of the second chapter of Miller's book. Moore's Principia (along with the work of Prichard, Ross, et al.) involves a defense of a particularly puzzling and problematic form of moral realism, namely non-naturalist intuitionism, and includes his famous Open Question Argument, which is the focus of Miller's chapter.
The second part of our story, and the remainder of Miller's book, begins with a backlash against Moore. Moore's non-naturalist intuitionism included the following views: that central components of moral language are indefinable, that moral facts can only be known as self-evident intuitions, and that moral properties are sui generis and not reducible to natural properties. A rejection of views of this sort gave rise to various forms of noncognitivism found in Ayer (who is Miller's representative of early noncognitivism), Stevenson, Hare, et al. that dominated English-language meta-ethics in the middle of the twentieth century. These philosophers rejected Moore's non-naturalist metaphysics and intuitionist epistemology as inconsistent with a naturalistic conception of the world. They also rejected moral realism because they thought Moore's OQA, or something similar to it, showed that realism was committed to defending those doctrines of Moore's that they found untenable (and perhaps incredible). The route out of the metaphysical and epistemological problems relating to morality, they thought, was to be found in a distinctive account of the nature of moral language. In particular, they argued that moral language is used to express our emotions and attitudes, or to prescribe certain actions for ourselves and others. However, the noncognitivists ended up running into all sorts of problems in accounting for our ordinary beliefs about ways in which moral language can be used, the possibility of moral knowledge, the existence of correct answers to more questions, and the objectivity of morality. In Miller's book, we see contemporary thinkers like Simon Blackburn and Allan Gibbard, who defend forms of noncognitivism, wrestling with these problems.
Still, their answers don't satisfy everyone, and some thinkers returned to a form of cognitivism: that is, they began to defend the position that ethical language purports to describe a realm of moral facts. And this brings us to the third part of our history of contemporary meta-ethics and of Miller's book: the backlash against (at least the earlier, cruder forms of) noncognitivism. All these cognitivists are concerned with finding a place for ethics within a naturalistic conception of the world. The first such thinker Miller discusses is John Mackie, who defended cognitivism but argued that all our ordinary moral claims fail to be true since there are no objective moral facts to describe. Others have a more sanguine view of possibilities for moral truth and knowledge, however, and they combine their cognitivism with a form of moral realism. Some of these thinkers argue that moral claims can be reduced to claims that are part of the natural and social sciences; others argue that moral claims aren't reducible in this sense, but that this doesn't impugn their naturalness since moral claims, like claims in other areas, can be natural without being reducible to the subjects of the natural sciences.
There are a couple of things that need to be said about what this book is not. First, despite its grounding in history, Miller's book isn't intended as a history of twentieth-century meta-ethical thought. Only the first two chapters of the books seem to be accurately described as history, and the work of major figures like Hare, Stevenson, Prichard, Ross, et al., isn't brought into the discussion at all. (For a brief discussion of historical matters concerning the relevant period, one might try Warnock's Contemporary Moral Philosophy.) Second, while the book does often present Miller's own judgment on the argument he discusses, it isn't a work that presents and criticizes the views of others as part of a defense of the author's own take on the issues at hand. (Introductions to meta-ethics of this variety can be acquired by reading works like Mackie's Ethics: Inventing Right and Wrong, Harman's The Nature of Morality, and Smith's The Moral Problem.)
Rather than a history or an argument for a distinctive position, Miller's book takes the form of a philosophy textbook: it is filled with brief discussions of several arguments for and against many of the positions that are defended by working meta-ethicists. The book does have some notable virtues for a textbook that takes this format. One such virtue is that the topics covered are well-chosen: the book covers most of the positions that you'll need to wade through the literature on these issues, along with the most important arguments for and against those positions. Another is that the coverage of some of these positions is quite broad for an introductory survey (the chapters on Blackburn's quasi-realism, Cornell realism, reductivist forms of naturalism, and McDowell's views are each at least thirty pages). Unlike many books of this sort, Miller's amounts to more than a laundry list of positions and arguments since many of the positions face similar problems and some thinkers appear as both proponents of certain views and critics of others. In addition, the prose is clear throughout, and this clarity isn't achieved through dumbing down the arguments. Finally, each chapter includes helpful suggestions for further reading in the relevant primary sources.
At the present time, this book is the one to pick up if you want a sense of the various positions being defended in meta-ethics. So the book is a perfect supplement for courses surveying contemporary issues in meta-ethics that are aimed at graduate students and advanced undergraduates.