From a conversation with Jen this morning:
R: Did you know the correct English plural for the word dwarf is dwarfs with an F?
J: Not …V-E-S, then?
R: No—although, when he was writing The Lord of the Rings, Tolkien, as an expert in linguistics, decided the word would be far more common in Middle Earth, so would have been corrupted through everyday usage. So he made a point of spelling the plural …V-E-S.
J: I dare say, after working that out, writing the rest of the book will have been a cinch.
Should really be dwarrows, of course.