The books that everyone must read Pretty sure the OP didn’t stipulate ‘philosophy books’ in particular or I would have answered differently.
For ‘must reads’ I have a fairly limited span in philosophical works but I would say:
- Critique of Pure Reason, because it is a challenge to read and requires concentration and study to get to grips with, as well as being one of the most important philosophical work ever produced.
- The Republic, because it is a great insight in ancient Greece and the origins of western philosophy.
- Being and Time, because it is awful yet a completely different and evasive style of writing that offers some nice points but ultimately outlines the border between meaningless drivel and brilliance.
- Something by Nietzsche, but do not start with Thus Spake … as he, like Heidegger, introduces a different style of writing to the world of philosophy.
- Philosophical Investigations, because it tackles important issues about language and language use, as well as being an amalgam of of ideas/thoughts.
I think these five give a pretty nice map of philosophy in terms of approach and style.
I still think Critique of Pure Reason is an essential read for anyone serious about philosophy. Second in line would probably be Being and Time. Not that I am a fan of Heidegger exactly, I just think it is important to see how obtuse philosophical texts can get and where, more or less, things went a bit off the rails.