The author raises the question of ethics in our overwhelmingly individualistic society. Unlike at least one other reviewer of this book, I didn't feel that it is a sermon in disguise to promote Singer's ideas about animal welfare, although it does receive mention several times. The book is a good synthesis of carefully chosen historically significant events and social analyses (e.g. the "Opening of Japan" at the Bay of Edo, and the culture of materialism, as well as foreign work ethics and so on). Singer's style is relaxed but not condescending. He argues in the end that although reality is relative, there is a sense in which that can be transcended to some degree. The idea is that as beings endowed with the gift of imagination, we are able to move in our minds beyond the relative reality, to observe that all beings suffer as we do personally. This slight glimpse of an objective truth that we permit ourselves to witness is a motivation and reason enough, argues Singer, for us to want to act ethically towards others.
The book is a mild philosophical romp, with some ideas in ethics, biology and evolution receiving mention. In no place is the book overwhelming.
I gave the book 4 stars because I don't think it is a masterpiece, and it doesn't make allowance for the fact that to some extent it is limited by its need to rationalise parts of human existence that don't necessarily warrant it or are capable of being rationalised. Nonetheless, one cannot write a book that accounts for the entire truth of human experience within the bounds of a light discussion.
On a practical level, the book is about 350 pages, just larger than the average paperback with a medium sized print (I know some books have discouragingly small letters (some Penguin ones for instance)).
I think it is very well worth reading, but don't expect it to be the best book on the topic of all time. Nonetheless, a good introduction, and even if you don't follow or buy into all the arguments Singer presents, there's plenty else you'll find out besides.