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"Being & Time" by Martin Heidegger. An intense, logical investigation of individual existence. A deeply nuanced philosophical take on death, being, and temporality. This is the book to grapple with your inevitable annihilation.

"A Thousand Plateaus" by Deleuze & Guattari. A psychedelic philosophy of complexity. Not exactly an airtight philosophical system, but it will blow your mind several times per page.

"Will to Power" by Friedrich Nietzsche, who you'll probably love if you like Rand.