I am really fascinated with Hume's discovery that an "ought" cannot be derived
I prefer to think of it as Hume's claim rather than Hume's discovery, since "discovery" implies the truth of what's discovered, and I think Hume was wrong, at least on what seems to me the most natural interpretation of what he says in the Treatise of Human Nature. But the interpretation is part of the problem; scholars disagree on what Hume meant. There's a magazine article on this topic, written by one of Hume's defenders, at this link. There's also a recent collection of essays, Hume on Is and Ought (Palgrave Macmillan, 2010), that goes into minute detail on the interpretation and evaluation of Hume's claim.