For me, it was "Behavior Trees in Robotics and AI: an introduction"

What conceptually helped me out was the idea that behavior trees (BTs) are hierarchical finite state machines (HFSMs). I read that and thought "woah!"

I've been fascinated by behavior trees ever since I learned that they were a big thing in Halo. It's charming to me to think that video games are able to help out in robotics, as the book pontificates on briefly.