The remarkable thing in the Book of Mormon is that only once in the whole book does the author stops cold to explain something. Do you remember where that is? It has to do with the monetary system. In the middle of the account of Zeezrom, the lawyer, the action suddenly stops cold. Why? Because the value of money is surely something that would have changed across the 300 or 400 years between Zeezrom and Mormon. That's a cultural difference Mormon would recognize. And, predictably, like any naive writer, he stops the action in order to explain the unfamiliar facts. But notice how he does it. There is no absolute reference. Apparently one of those words or values in his list still meant something in the fourth century A.D. He didn't think to tell us exactly what you could buy with a senon, and certainly not in any terms that would mean anything to an 1820s reader, because Mormon must have considered the value of some element in the monetary system to be obvious. So even in the one place where we do get an "expository lump," it's handled exactly as a writer from the alien culture would have handled it, and utterly without reference to the 1820s.
From "Artifact or Artifice" by Orson Scott Card.