There are numerous books on the topic. I recommend the recently published, excellent book by John Deigh, An Introduction to Ethics (Cambridge University Press, 2009).

The question reminds me of Schiller's lines. Scruples of Conscience I like to serve my friends, but unfortunately I do it by inclination And so often I am bothered by the thought that I am not virtuous. Decision There is no other way but this! You must seek to despise them And do with repugnance what duty bids you. These lines are often cited as an objection to Kant's account of moral worth; Frederick Beiser challenges the standard reading of these lines in a discussion of the relation between Kant and Schiller in Schiller as Philospher: A Re-Examination . I want also to add a few remarks about Kant to Matthew's perceptive response. First, a point about the examples in the Groundwork . Kant introduces those examples in order to isolate the moral motive, in order to explicate the concept of a good will, which Kant introduces in the first sentence of the body of the Groundwork : "It is impossible to think of...