About the book (from Wikipedia):
An Enquiry Concerning Human Understanding is a book by the Scottish empiricist philosopher David Hume, published in English in 1748. It was a revision of an earlier effort, Hume’s A Treatise of Human Nature, published anonymously in London in 1739–40. Hume was disappointed with the reception of the Treatise, which “fell dead-born from the press,” as he put it, and so tried again to disseminate his more developed ideas to the public by writing a shorter and more polemical work.
The end product of his labours was the Enquiry. The Enquiry dispensed with much of the material from the Treatise, in favor of clarifying and emphasizing its most important aspects. For example, Hume’s views on personal identity do not appear. However, more vital propositions, such as Hume’s argument for the role of habit in a theory of knowledge, are retained.
This book has proven highly influential, both in the years that would immediately follow and today. Immanuel Kant points to it as the book which woke him from his self-described “dogmatic slumber”. The Enquiry is widely regarded as a classic in modern philosophical literature.
An Enquiry Concerning the Principles of Morals is a book by Scottish enlightenment philosopher David Hume. In it, Hume argues (among other things) that the foundations of morals lie with sentiment, not reason.
An Enquiry Concerning the Principles of Morals is the enquiry subsequent to the Enquiry Concerning Human Understanding. Thus, it is often referred to as “the second Enquiry”. It was originally published in 1751, three years after the first Enquiry. Hume first discusses ethics in A Treatise of Human Nature (in Book 3 – “Of Morals”). He later extracted and expounded upon the ideas he proposed there in his second Enquiry. In his short autobiographical work, My Own Life (1776), Hume states that his second Enquiry is “of all my writings, historical, philosophical, or literary, incomparably the best.”