When I was a highscooler, I was really interested and math, but didn't like computer science that much: it seemed too practical and real-world grounded for me, and I liked abstract stuff. What changed my mind was
I liked how computer science turned out to be rich with mathematics and how it could be used to give a rigorous treatment to philosophical questions.
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And after I become a university student, the most important book for me for a few years has become
"Computational Complexity: A Modern Approach" by S. Arora and B. Barak.