Ecce Homo by Friedrich Nietzsche. I picked it up knowing nothing but Nietzsche was the progenitor of the saying 'that which does not kill you makes you stronger' (which is a radically misunderstood saying that is supposed to sound naive and stupid). Then I read the chapter titles. "Why I Am So Wise", "Why I Am So Clever", "Why I Write Such Great Books". I had to read it. It sounded ludicrous from the table of contents alone. And so it was. I later found out that he wrote the book while in the end stages of syphillis, mostly insane. It did, however, clue me in to a whole different kind of thinking by introducing me to philosophy and how problems could be tackled that can't be addressed through more analytical means. Plus, it is full of a great many laughs simply due to the absurdity. The only thing I can recall actually making sense and being true was a bit about how to be most like ones parents is the greatest disgrace because it indicates that humanity has stagnated and not advanced for your generation at least.
And though I don't care for it now, Ayn Rand's works, Atlas Shrugged and The Fountainhead, had great impact on me at the time I read them. I went on to read all of Rand's non-fiction philosophical writings as well. It was thanks to her that when I attended college, I majored not just in Computer Science, but also in Philosophy. And I credit that education with granting me a tremendous amount of insight and philosophical confidence. Today I would vigorously disagree with many of Rand's shortsighted conclusions, and even with her physically incorrect epistemology, but she did have a grand idea after all, to derive all of philosophy from first principles. Shame she started with the wrong principles and A is indeed often not A but a superposition of A and B. Beyond that, the modern Objectivist community seems to lack a great deal of the intellectual mettle that I think she herself had, and they seem incapable of letting go of the seemingly helpful flawed evidence of the past and embracing the more sound modern evidence of the benefits and evolutionary advantage of empathy and such. I could go on for hours about how Rand actually DID account for things like charity and supported them, but it's a nuanced discussion and not for a thread like this.
Also, the hardest book to read I have ever read, Comprehensive Mathematics For Computer Scientists. It derives mathematics from set theory on up. It is written like a single books-long (there are multiple volumes) academic paper and if you're not accustomed to reading such information-dense material, it might well be impenetrable. It took me days per page to fully comprehend, often with breaks in between. But it gave me interesting perspectives on at least functional programming and infinity and how to prove things.