It isn’t easy to find material in metaethics that is pitched to introductory students. My own book, Whatever Happened to Good and Evil? (Oxford University Press, 2004), is designed for readers with no prior philosophical training. Alexander Miller’s Contemporary Metaethics: An Introduction, 2nd ed. (Polity, 2013) is written for those with a fair bit of philosophy under their belt but is a reliable survey of most of the major theories in the area. A historically informed survey of views on the topic is given by Stephen Darwall in his Philosophical Ethics (Westview, 1997).
Important and fairly accessible work in this area is offered by J. L. Mackie in the first chapter of his Ethics: Inventing Right and Wrong (Penguin, 1977). Michael Smith’s The Moral Problem (Blackwell, 1994) provides an excellent way in to the many problems in metaethics. Smith has also written an introductory article that highlights a number of the themes of his book: “Realism” in Peter Singer, editor, Ethics (Oxford University Press, 1994), pages 170–176.
For a wide-ranging collection containing many classic and contemporary writings in metaethics, along with a dozen substantial introductory essays designed with the student reader in mind, see Terence Cuneo and Russ Shafer-Landau, editors, Foundations of Ethics: An Anthology (Blackwell, 2006).