It might be read as an introduction to Ethics, but it isn't one. It is rather one of the most important works in 20th century ethics.
Mackie's book was revolutionary since being the first one to combine anti-realism (no objectively prescriptive values in the world) with cognitivism (the meaning of ethical statements can be true or false). Most of the previous anti-realists were anti-realists mostly implicitly, only because of being non-cognitivsts. Mackie has a different view which in my opinion is much more closer to the truth. The book also contains his error theory (people have a disposition to see their value judgments as objective). While the reviewer cdtreyer as the mainstream tradition have concentrated on Mackie's error theory I think it is much less important than the denial of the objective values and the justification of the role of morality in quasi-contractual terms.
Mackie's views on positive morality are justified by quasi-contractual (he discusses Plato's Protagoras, Hobbes and Hume) means and would combine very well with evolutionary perspectives. The discussion on the content of normative views is just a brief sketch, but this isn't really what this book is about anyway. Anyone who claims that the contents of the first part of the book undermine the contents of the second should read chapter 5 again and again and again. That there are no objective values in the world does not mean that there can't be right or wrong - it simply must be (or rather already has largely been) invented and constructed.
If you are interested in ethics you simply need to read this small, but important book which, while not being an introduction is still quite simple and very elegantly written. Besides the main content you will also get to read a great discussion on the meaning of the good (in debate with the classical Geach-Hare discussion found in Philippa Foot's "Theories of Ethics"), discussion on the is-ought problem and its flawed Searlean solution (also found in Foots collection), a chapter on univerzalisability of moral judgments (contra Hare) and on the frontiers of ethics: voluntary actions, determinism, law, politics, religion.