Tbh I don’t know about read because I~~ can’t read~~ don’t study psychology, but I heard it first from Daniel Meckler, an ex-psychiatrist. He explains it well and it comes up in every video, so in his eyes it’s the most important thing to grasp about human psychology for your own sake or for the sake of practicing it.

You could scroll through his page and see what topic seems most interesting because you’re almost bound to hear it in one of them