The Legal Theory Bookworm recommends The Sources of Normativity by Christine M. Korsgaard. Here is a description:
Ethical concepts are, or purport to be, normative. They make claims on us: they command, oblige, recommend, or guide. But where does their authority over us come from? Christine Korsgaard identifies and examines four accounts of the source of normativity that have been advocated by modern moral philosophers–voluntarism, realism, reflective endorsement, and the appeal to autonomy–and shows how Kant’s autonomy-based account emerges as a synthesis of the other three. Her discussion is followed by commentary from G.A. Cohen, Raymond Geuss, Thomas Nagel, and Bernard Williams, and a reply by Korsgaard.
And from the reviews:
"This book is destined to replace Kant as the ultimate formulation of Kantian ethics. It should be required reading for any philosopher and should be in every library." W.F. Desmond, Choice
"This is a book anyone working in ethics should have on the desk. It is provocative and makes original and major contributions to a defense of a Kantian ethic. The historical developments of the various strands of thought are traced out in clear and helpful style. Korsgaard’s writing is itself engaging and clear and her arguments forceful and for the most part compelling. This book constitutes a major advance in ethical theory." L. W. Colter, Review of Metaphysics
A great book and essential for anyone interested in contemporary Kantian ethics.