Many of the snippets here talk about what was in individual commonplace books and how their contents indicated what was admired. What about the negative image of all the things which were excluded?
What things did people read which they didn't commonplace? What does that say about them? Their times? The way they thought?
Could their commonplacing be compared with their marginalia to show what rose to certain levels of interest and what didn't? Digging through the records to find and verify marginalia may be incredibly tedious in comparison with specifically kept things.
Some of my own fleeting notes, which I keep because it's easy, may show some interesting things about the way I think in comparison to those things upon which I expand, keep, and value more for my work and my worldview.