Hegel's Phenomenology of Spirit.
Kant's Critique of Pure Reason.
Hofstadter's Godel, Escher, Bach. I also recommend Zizek's The Sublime Object of Ideology, but it's dense, so his film The Pervert's Guide to Ideology is a good stand in.
Hofstadter's book is brilliant, funny, delightful - a real treasure.
He's not a major philosopher like Kant, but he is much more than a charlatan or provocateur.
To his credit, I probably wouldn't have picked up his serious work if his antics hadn't first put him on my radar many years ago.