Finding out what’s true and what’s unknown. The latter is important; all sciences except mathematics operate with some degree of uncertainty. (In mathematics, once something is proven and other mathematicians agree the proof is sound, it stays proven – in mathematical terms, a theory becomes a theorem.)
When something unexpected happens! It usually means something is broken or you made a mistake, but sometimes it can be really exciting because you might be about to learn something that we didn’t know before.
Research methods. I love research methods, and social science methodology books. They impose a totally superficial order on a chaotic universe, and give me the sense I may actually have the tools to solve some of the problems that exist in the world.