My favorite book (impossible to pick one, btw... maybe give me 10 choices) but I'd say "The Great Gatsby". On the level of words, it is one of the most beautiful reading experiences one can have: it's poetry. On the level of structure, it has one of the most elegant, simple, clear plot constructions I have run into. It showed me how a great writer uses focus to tell a universal story.
And on that level, story? It broke my heart. At first I was repulsed by several of the characters, and the lives they led. By the halfway point, I had come to recognize in those same characters the people in my real life, and really, to see parts of myself, laid open to view.
By the end, I realized, I saw, for the first time in my life, how most people, most of us, live our lives reacting to imaginary futures and selectives pasts and all sorts of phantoms and projections, and just... miss the chance at living a real life.
There is absolutely nothing "metaphysical" about "The Great Gatsby". But it is remarkably human and humane, and opened the doors of my life toward searching for the More.