@tsarstepan,
Singer is my favorite ethicist, too. If I could only read one book by him, it would be
Practical Ethics. In the first chapter of
Practical Ethics, he explains his (Utilitarian) philosophy. Then, throughout the rest of the book, he applies it to controversial topics such as the ethics of killing and inflicting pain on animals, euthanasia, and helping the poor. Refreshingly, and I think correctly, he states right out that sex does not pose a single unique moral problem, and that he won't say another word about it. It's a good book, which I warmly recommend.