Orin Kerr & Sasha Volokh have an interesting exchange going on the is-ought distinction & the nature of (constitutional) law. There are four posts:
Kerr-Confusing "Is" and "Ought" in Constitutional Law
Volokk-Is the "is"/"ought" distinction so clear in constitutional law?
The exchange is well worth reading–and it points, I think, to the continued importance of mastering the core literature in the what-is-law debate, roughly the Hart-Dworkin-Raz-Coleman-Shapiro line of development. If you read the exchange, you will see echos of Holmes’s prediction theory of law and its critics and refracted versions of exclusive and inclusive legal positivism, with perhaps a dash of Dworkin.