Thank you for the quick response. I am a rank beginner, about a hundred pages through D'Ooge, and haven't encountered the double nominative yet by name. I never thought of a line like "Lesbia dea est" as being a double nominative, I just saw that as a use of the appositive, with "dea" being a description of Lesbia.
What confused me with "Et verbum caro factum est" is that the Word is MADE flesh, a change is effected, clearly "caro" does not just describe "verbum." Now it makes sense, thanks again!