An Enquiry Concerning the Principles of Morals is a 1751 work by David Hume, which developed the ethical philosophy first expounded in his Treatise on Human Nature. It is often known as as the Second Enquiry, to distinguish it from an Enquiry Concerning Human Understanding.
While the classification of Hume's ethical position remains controversial, it can be described as sentamentalist, in rejecting the view that moral judgements can be founded on reason alone.
Free online texts
Early Modern Texts: An Enquiry Concerning the Principles of Morals, adapted and translated into modern English, by Jonathan Bennett. PDF format.
Gutenberg: An Enquiry Concerning the Principles of Morals. Multiple formats.
Internet Archive: An Enquiry Concerning the Principles of Morals (1912). Multiple formats.
University of Adelaide (Internet Archive): An Enquiry into the Principles of Morals. Multiple formats.
Wikisource: An Enquiry Concerning the Principles of Morals. HTML and other formats.
BBC Radio 4 In Our Time: David Hume. Melvyn Bragg with Peter Millican, Helen Beebee and James Harris.
Internet Encyclopedia of Philosophy: David Hume (1711-1776)
Librivox - Public domain audiobook: An Enquiry Concerning the Principles of Morals
PhilPapers: Hume - An Enquiry Concerning the Principles of Morals - bibliography with open access option.
The Great Conversation: Further reading at Tom's Learning Notes
David Hume: A Treatise of Human Nature.
Thomas Hobbes: Leviathan - Hume argues against Hobbesian egoism.
Mandeville: The Fable of the Bees - a more overtly cynical version of the egoist position, which Hume likewise rejects.
Francis Hutcheson - an earlier version of the sentamentalist position, differing in significant ways from Hume's.