Conglomerate asks, "If You Could Choose Only One Book For Prospective Law Students, What would it be?"
I would answer F.A. Hayek, Law, Legislation, and Liberty, Volume 1, Rules & Order. In this book, Hayek sets out the "spontaneous order" theory of the common law. I like it because it is implicitly a defense of traditional legal conceptualism, to understand the elegance and wisdom of the common law, and to help appreciate the importance of the common law and the rule of law as the foundation of freedom and prosperity in the Anglo-American world. The deep and tacit wisdom embedded in the common law is often lost in the modern legal realist perspective that dominates the academy today.
But then again, "Hayek" is my answer to every question like this, prospective law student or otherwise.
As a close second, and certainly more enjoyable than Hayek, I would recommend Thomas Sowell, A Conflict of Visions, which I have commented on previously. Sowell's book helps us to understand the fault lines in much of public debate today, and none moreso than disputes over differing approaches to law in society today, such as the role of economic analysis, the debate over originalism versus non-originalism in constitutional interpretation, and the importance of public choice theory in understanding law. I think that understanding these rough fault lines helps to recognize the unarticulated starting points that lie behind many modern legal debates.