The Legal Theory Bookworm recommends The Concept of Law by H.L.A. Hart. This is the new Third Edition, with an introduction by Leslie Greene. Here is a description:
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Fifty years on from its original publication, HLA Hart's The Concept of Law is widely recognized as the most important work of legal philosophy published in the twentieth century, and remains the starting point for most students coming to the subject for the first time.
In this third edition, Leslie Green provides a new introduction that sets the book in the context of subsequent developments in social and political philosophy, clarifying misunderstandings of Hart's project and highlighting central tensions and problems in the work.
In my opinion, the Concept of Law remains essential reading for every legal academic. It is not uncommon to encounter very sophisticated members of the legal academy who have never read Hart's book. Even if you are skeptical of the value of the nature-of-law debate in contemporary "analytic" philosophy of law, you need to understand Hart, whose thought had a fundamental shaping role on debates far afield of the dispute between among legal positivists and between them and natural lawyers and interpretivists.