Objectivism: The Philosophy of Ayn Rand by Leonard Peikoff was one of the first philosophy books I read, back when I was a teenager. It persuaded me that philosophical questions had objective answers and provided a coherent framework that I could use to approach philosophical issues with.
The book is a distilled presentation of the essential theoretical structure of Objectivism. It starts at the foundation with metaphysics, then proceeds to concept formation, epistemology, the good, the moral virtues, politics, capitalism, and aesthetics. Each topic is explained clearly and linked to the foregoing and previous.
Peikoff’s presentation of Rand’s philosophy is very engaging and persuasive. I recommend it if you’re interested in a compelling, systematic challenge to your worldview.