There are a huge number of elementary introductions to ethical theory and choosing between them is very much about finding one that suits, but for comprehensiveness and clarity, Pojman, L. P.1990. Ethics. Belmont: Wadsworth, is worth looking at. Also, for a particularly comprehensive treatment (including classificatory tables, summaries, chapter by chapter 'recommended readings', and a glossary), Bond, E. J. 1996. Ethics and Human Well-Being: an introduction to moral philosophy. Oxford: Blackwell, could prove useful.

As a starting point for more advanced study, anthologies are the most convenient source and they are concentrated on in the following listing.

La Follette, H. (ed.). 2000. The Blackwell Guide to Ethical Theory. Oxford: Blackwell. Specially written sections by different authors providing an introduction to the main varieties of ethical theory.

Jacobs, J. 2002. Dimensions of Moral Theory: an introduction to metaethics and moral psychology. Oxford: Blackwell. As the sub-heading suggests, it focuses on those complex and controversial issues relating to the objectivity or otherwise of ethical judgements and the motivation for ethical behaviour (traditionally labelled 'metaethics' and 'moral psychology' respectively) rather than the accounts of different sorts of 'normative theory' (consequentialism, Kantianism, etc.) more usual in introductory texts (as in Bond above).

Sterba, J. P. (ed.). 1998. Ethics: the Big Questions. Oxford: Blackwell. A selection of modern and classic readings on the nature of morality and different theoretical approaches to it.

Singer, P. (ed.). 1994. Ethics. Oxford: Oxford University Press. A selection of modern and classic readings (mostly by way of brief extracts) drawn from the social and natural sciences as well as philosophy.

Singer, P. (ed.). 1993. A Companion to Ethics. Oxford: Blackwell. Specially authored chapters providing a comprehensive coverage, with part IV on different theoretical approaches and (possibly) part VI on the nature of ethics being the most useful for teachers of business ethics (along with a chapter on business ethics in the 'applications' section).