I chose, in no particular order:
1) Spinoza's Ethics
2) Thomas Mann's The Magic Mountain
3) Fyodor Dostoevsky's The Brothers Karamazov
4) Mary Shelley's Frankenstein
5) Friedrich Nietzsche's Thus Spoke Zarathustra
If I could be permitted a runner-up (or two), I'd also list Søren Kierkegaard's Works of Love, and Gilles Deleuze and Félix Guattari's What is Philosophy?
What about you?