The full title of this work is Animal Liberation: the Definitive Classic of the Animal Movement, by Peter Singer. Keep in mind that when the book was first published in the 1970s, "animal rights" was almost unheard of. The book was revised in the 1990s, and is now a classic among PETA supporters and all who support animal rights.
First, Singer wants to put to rest the assertion that "animals shouldn't have rights just because they are animals." The way that Singer proves this wrong is by insisting that is a tautology: just like saying that women shouldn't have rights just because they are women or Native Americans shouldn't have rights just because they are Native Americans. Singer says that just like the first example is sexism and the second example is racism, the "animals shouldn't have rights because they are animals" example is called "speciesism." This new term defined means discrimination based on species. Singer also believes that once the other two discriminatory practices are knocked out, it's easier to knock out speciesism as well.
Next, Singer takes on the idea that if speciesism is not sufficient for denying rights to animals, then why are animals treated the way they are? Singer goes through the usual list: they act on instinct, can't build things, can't write, etc. Then singer begins to compare the higher forms of the animal world (dolphins and chimps) with the lower human species: severely handicapped children. Singer's point is that animals have higher capabilities, yet they don't have as many rights as the severely handicapped child.
Then singer puts forth the best test for rights: suffering. If a living form can suffer, then its rights can be violated. He puts this in contrast with plants and rocks (that don't feel pain). Singer then "shows" how animals feel pain. One example is the reaction of a steer to a cattle prod. Singer then explains the animal's neurological response to pain: similar to humans.
Now Singer is set to note current animal treatment and see if rights are infringed upon. This is the part of the book that a reader might expect to find. The realistic description of factory farming of animals as inhumane. Many animals are included: cows, pigs, chickens, etc. A further addition in the latest version is the use of antibiotics in the meat, pumped into the animals while they are still alive.
In conclusion, I find it interesting to use this theory applied to the human fetus. Singer, I guess, would attest that the fetus has rights when it can feel pain. Pretty early on. An interesting idea to a pro-PETA and, yet, pro-choice supporter!