I just finished reading Rothbard’s Ethics of Liberty a few hours ago. In the beginning of the book he says something like, “It is not the purpose of this book to go into detail proving the existence of natural law.” (That’s just a paraphrase, I might be able to find the exact wording if you want.)
The reason I’m here is because I’m looking for a book that does go into detail about it.
Hmm I know of no book that is just specifically natural law alone. There is Locke, Hugo Grotius, Francisco Suárez, Samuel von Pufendorf, Aristotle to a degree.
Look up Norms of Liberty and Liberty and Nature by Douglas Rasmussen and Douglas den Uyl and Rational Man, For an ontology of morals and Human Rights: fact or fancy? by Henry Veatch.