Points taken, Brian.

One thing that I mentioned in my original thread - which I don't think has been touched on - is the change of key range over time. The old general tune books that I have (and I exclude the ones aimed at a particular instrument like the small pipes) do seem to contain tunes with a wider span.

I suppose that "sessions" as we know them now are a comparatively more recent phenomenon, and the tune books - particularly the family tune books - were collections used mainly for accompanying dances and social events.