His later work prompted me to learn Order Theory, which has turned out to be useful for all sorts of things. Also quite closely related to Category Theory which I wouldn't have had much chance of grokking without first understanding Order Theory, I suspect.

I also used LaTeX heavily in the 80s so was surprised to see him pop up as a genius of distributed systems later (although that work was published much earlier it didn't get much exposure until the 90s). Like "oh that guy must be _really_ smart to excel in two quite different fields".