Ronald Dworkin, Taking Rights Seriously (1978) (I'd rather include Ronald Dworkin Taking Rights Seriously than Ronald Dworkin Law's Empire (1986) if I had to choose, because while I think there's more to his constructive account of law than, say, Leiter does (and that Dworkin's account is often misunderstood), it seems to me that his most important contributions are the more negative ones found in TRS rather than the positive ones in LE; suggested by Matt). NOTE: If we're talking philosophy of law, Leiter's list seems better to me. Dworkin's shorter pieces in TRS & other books might be more important, but they aren't his most important contributions to the philosophy of law. Law's Empire is definitely the thing to include. Hart, Fuller, Finnis & Dworkin are the biggies; suggested by Chris.