Impossible to pick just one. I'll give a few that haven't appeared yet: Fiction: Harry Potter and the Methods of Rationality (Eliezer Yudkowsky) Non-fiction: Rationality - from AI to Zombies (Eliezer Yudkowsky) Probably no books have impacted me quite as much as these two, barring early-childhood books that have impacted me from a young age. Fiction: Worm (by Parahumans) Best "superhero" fiction ever created. Just on…
I highly recommend listening to the GraphicAudio adaptation of this series. It's what got me into audiobooks.