Laws limiting the height of your fence are debated in the following way.

1. First, the benefits and harms to you and your neighbors are considered.
2. These considerations are weighed against the relative costs to the rest of the community.
3. Rules are agreed upon that take everyone’s interests into account.

This is a moral exercise, because to debate something is to engage in normative deliberation. Indeed, your only mechanism for social appeal is moral: “This is the right thing to do” for some reason, such as justice, fairness, harm reduction, or whatever.

If you pass a law protecting rare valuable books, then you also need to decide which books are protected.

You’re right. Laws are constrained by pragmatism. It turns out, figuring out which books are important is analogous to figuring out which natural habitats should be legally protected from construction, etc.