The Legal Theory Bookworm recommends Judging Positivism by Margaret Martin. Here is a description:
Judging Positivism is a critical exploration of the method and substance of legal positivism. Author Margaret Martin is primarily concerned with the manner in which theorists who adopt the dominant positivist paradigm ask a limited set of questions and offer an equally limited set of answers, artificially circumscribing the field of legal philosophy in the process. The book focuses primarily, but not exclusively, on the writings of prominent legal positivist Joseph Raz. Martin argues that Raz's theory has changed over time and that these changes have led to deep inconsistencies and incoherencies in his account. One reoccurring theme in the book is that Razian positivism collapses from within. In the process of defending his own position, Raz is led to support the views of many of his main rivals, namely Ronald Dworkin, the legal realists, and the normative positivists. The internal collapse of Razian positivism proves to be instructive. Promising paths of inquiry come into view and questions that have been suppressed or marginalized by positivists re-emerge, ready for curious minds to reflect on anew. The broader vision of jurisprudential inquiry defended in this book re-connects philosophy with the work of practitioners and the worries of law's subjects, bringing into focus the relevance of legal philosophy for lawyers and laymen alike.
Sure to be controversial!