If you just search customary law nearly every book that shows up is about decolonizing law, or international law, neither of which interest me. The two general relevant texts I found were ``The Philosophy of Customary Law'' and ``The Nature of Customary Law'' both of which have a single poor review but I may look into in any case. (I only have access to the latter) It may be that the best approach is simply to read early medeival barbarian legal codes.
Unforuntately the codification of customary law by the state seems to negate much of what makes it customary law, and in fact in Europe this typically occured with the spread of Christianity along with writing and Latin influences more broadly. Our lens to European customary law is then colored by either mythical stories or by the early codification by Latin and Christian influenced governments, and more often than not both.
On the plus side I've found the name of something similar to what I was looking for. It's called legal positivism, a relatively recent adventure from the 18th and 19th centuries. So for the moment I'm going to read more about this. It builds its self interestingly in the analytic tradition rather than in philology, and it seems like it may be missing something, but it's a start.