As I suggested long ago in my review of Peter Singer's _Animal Liberation_, while I applaud Singer for pointing out numerous ways in which our treatment of animals could be improved, I don't find his "utilitarian" ethical arguments very persuasive.
But Tom Regan's now-classic book -- this one -- is a different story. This is a tour-de-force of ethical argumentation that makes the titular case about as well as it's ever going to be made. Regan doesn't simplify any issues and he's very much alive to fine ethical nuances. And he sets out his case with both rigor and vigor.
Probably most of us won't have any problem agreeing that at least some nonhuman animals are conscious, but there _have_ been people who have denied it (most famously, Rene Descartes). So for completeness, Regan begins with a careful discussion of the question. Avoiding simplistic answers and over-eager claims about research on e.g. animal language, he mounts a solid case that at least some nonhumans do possess consciousness.
(Some of his arguments are a bit weaker than he thinks they are, although I still agree with his conclusions. For example, he argues that possession of language skills can't be an indicator of consciousness because human infants are presumably conscious before they acquire a language; how else, indeed, would they acquire it? But this shows only that _present_ possession of linguistic ability isn't a necessary condition of consciousness; it doesn't show that the ability to _learn_ a language isn't such a condition. As I said, though, I agree with his conclusion; I'm merely criticizing the way he gets to it.)
The remainder of the book is a wide-ranging discussion, not just of animal rights, but of ethics generally. Even aside from Regan's nominal topic, the volume could serve as a fine introduction to ethical thought in general. (Among its many highlights: a short refutation of Jan Narveson's "rational egoism" that could double as a refutation of Ayn Rand's even sillier version.)
In the end, what this gets us is a careful case for regarding mammalian animals which are at least a year old as possessors of "rights." (Regan also argues that for other reasons, we could and should want to extend "rights" to other animals; he has limited his discussion to mammals in order to keep to what he takes to be a fairly clear-cut case.) These "rights" do not, he holds, trump every other ethical consideration under the sun; in particular, in emergency situations in which either (say) a human being or a dog (or a million dogs) must be killed, we should kill the dog (or dogs) every time. These "rights" are _prima facie_ moral claims -- strong, but not indefeasible.
What I think Regan has successfully shown is that living beings don't have to be moral _agents_ in order to count in our moral deliberations. And with most of what he says on this subject, I heartily agree; in particular I think he has made just the right distinction between moral agents and moral patients, and correctly argued that moral patients have _some_ sort of "right" to consideration.
I cannot, however, follow him _quite_ all the way to his conclusions -- for example, that we are morally obliged to be vegetarian and to refrain from using animals in all scientific research. Mind you, I've been a vegetarian myself and I think there _are_ good reasons for avoiding meat; I just don't think they're morally conclusive. I agree completely that many current practices are inhumane, and I also agree with a point Regan argues repeatedly: that moral limitations on what we can do with animals do _not_, as such, interfere with the operation of the free market. But I'm still not altogether sold.
(The problem -- to put it briefly and inadequately -- is that I think Regan assigns too much to moral _patients_ in the way of "rights." I'm not persuaded that in order to have a "right," it's enough that someone else could make a moral claim on your behalf. In other words, I disagree with Regan's contention that moral agents and moral patients are entitled to exactly the _same_ sorts of moral consideration.)
I don't, however, mind admitting that Regan has changed my mind on some points and may yet change my mind on others. If I ever _do_ change my mind on this last point, he will be in part responsible.
And at any rate I highly recommend this volume to any readers interested in the topic of animal rights. Moral reasoning doesn't get any better than this.