The most important books are often the least well known. That is certainly true of Thomas E. Davitt's, The Nature of Law, which at first glance seems to be a misnomer. This excellent book is a detailed historical and philosophical study of the major thinkers who addressed the problem of the nature of law from the point of view either of the primacy of the will or the primacy of the intellect.
While this brief summary may cause the eyes of most who read this post to glaze over, there may be no other legal issue more far-reaching in society today. In short, how one sees law--from the point of view of the primacy of the will or the primacy of intellect--determines whether one is a positivist or whether one embraces the natural law tradition.
To the positivist, all that is necessary for a law to be legitimate is that it be posited (enacted) by a proper authority. Obligation is the principal intrinsic effect of law. Thus, there is never a necessary connection between law and morality. The contrary view is that law is an act of the intellect and, thus, laws oblige in conscience because they simply specify means that lead to an end that man, by his very nature, is already obliged to work toward. In this view, for a law to be legitimate it must always be moral in its end and in its means. Therefore, there is always a necessary connection between law and morality.
Admittedly, this whole discussion is somewhat complex and rooted in a philosophical argument seldom considered these days. But its effect can be made concrete with a couple of questions. Are laws legitimate simply because an authority wills (by enacting a law) to oblige those under it to this or that? Or, is an authority responsible to enact laws that are moral in their ends and their means in order to oblige those under it to this or that? These are likely the most important, and ultimately the most practical, questions in law today. And, Davitt's outstanding historical and philosophical work, The Nature of Law, is perhaps the most comprehensive introduction to the subject. Unfortunately, I believe it is out-of-print and must be secured on the secondary market.
One final note. Beware, those who take the view that by the very fact that God wills it, an act (or law) becomes good and just. This was the view of William Ockham, who said: "Hatred of God, stealing, committing adultery, are forbidden by God. But they could be ordered by God; and, if they were, they would be meritorious acts." At its root, and in principle, this view is no less positivist, and no less destructive, than the view that the will of the State is the source of obligation . If Ockham could see where his views have taken us, he might have seen the issue of primacy of the will or primacy of the intellect differently.