Beyond Good and Evil: Prelude to a Philosophy of the Future - Friedrich Nietzsche

Nietzsche's work, unlike Plato's Symposium, does not read like a story, with a plot, characters, action and dialog. It's a collection of anectodes, philosofical thoughts, not necessarily easy to grasp and sometimes quite contradictory.
It requires some effort, but it's extremely rewarding. Nietzsche was a very refined genius, and regardless of whether we agree or not with him, following his logic and reasoning is extremely fascinating.
This book digs into topics that he already dealt with in his masterpiece "Thus spoke Zarathustra". It's about the relation between strong and weak individuals, morals, law.
Extremely interesting, and in a way still very applicable today.