Plug read, opinions needed
A couple more views.
Both guys have worked a lot with tuning race engines on dynos and at the track.
Bruce "Shrinker" Robertson: The direct link I posted in that thread still works. PDF format.
racingfuelsystems-Shrinker on Spark plug reading
This goes in pretty deep, with each example discussed in context with the specific engine and other measurements.
Larry Meaux on the basics: HTML (web format)
sparkplugreading
Note - I don't know how old this is, and so Larry may have changed his conclusions since it was posted.
edit: Direct link to pdf available as FABO attachment later in this thread