This books is widely credited for setting off the animal "rights" movement. Singer really brought the issue into the public arena, and caused people to question their presumptuous beliefs - this book was first published in 1975, and sparked off the writing of literally hundreds of other books about animal "rights". He describes our traditional view of animals as "speciesist" - arbitrarily discriminating simply on the basis of species - comparable to sexist or racist views. In the book he argues rationally and convincingly for animal "rights". Although a work of philosophy, the book is written to be easily accessible to the lay-person.
The book explains why we must extend our moral principles to other animals, describes the cruelty occuring in laboratories and factory farms, tells how and why we should become vegetarians, gives a short history of our views of animals and where they came from, and refutes common arguments against animal "rights".
In the 1990 preface to this book, Singer writes of its arguments "I have lectured on them, given talks to conferences and philosophy department seminars, and discussed them at length, both verbally and in print; but I have come across no insurmountable objections, nothing that has led me to think that the simple ethical arguments on which the book is based are anything but sound. It has been encouraging to find that many of my most respected philosophical peers agree with this view - so many, in fact, that in reviewing the revised edition Colin McGinn, who holds a distinguished chair of philosophy at Rutgers University, described the ethical core of the book as, in theory if not in practice, 'a won argument'".
If you find that hard to believe, then read the book and see if you can refute its claims!