Ask HN: Going from Developer to Manager. What should I know or learn?

After reading (and resonating quite a bit with) "Developer Hegemony" by Erik Dietrich, which was one of the recommended books in the "what did you read in 2018" posts, I think about steering my career to a manager instead of a developer position.

For all those of you who've done the same or planning to do the same: What are the most important skills or what's the most needed knowledge in such a position? Where can I learn it? What should I read?