I read Peter Teal's Hand Woolcombing and Spinning. Teal recommends dying wool and then combining it (using wool combs, naturally) with undyed wool because, said he, the result feels better than wool where the whole amount is dyed.
I wondered if this was so.
Then I got to handle someone's skein that they had dyed and then over-dyed with black. The colour was very rich and deep. The feel of the wool was, well, I don't really have a vocabulary for judging skeins. But I can tell you it started out as the same white superwash merino as was in my sixteenth skein, which I knew by feel, and this felt different. Less slick.
I'm not saying this was bad or anything. Nor am I drawing conclusions based on an abstract book rule and a single skein. It was just interesting to make the connection.