Existentialism Is a Humanism

On the philosophical side, I sugget "Existentialism Is a Humanism" by Sartre: it explains in a clear and simple way the idea of Existentialism, which is a great philosophy based on strong principles, and a key idea in the modern philosophical debate. It somehow changed my life, and made me think a lot more about life principles that can actually guide one's life.

I think the quality of advice you're getting on this thread is very mixed. Some of the books mentioned are absolutely great-- like "Being and Time", or Husserl's "Logical Investigations"-- but incredibly difficult to read unprepared. If you were ready to be reading them, you wouldn't be asking the question here. The two books you mention in the question (Common Sense and "On the Fourfold Root") are both quite readable, as are a few of the primary texts mentioned in the responses ("Existentialism is a Humanism", "Thus Spoke Zarathustra") but in general, I'd have to suggest you stay away from primary texts at first. In other words, begin by reading works about philosophy, rather than works of philosophy. A single-volume work on the history of philosophy will give you the broad scope (for example, Durant, or Russell, or "Sophie's World" if you prefer fiction), but will tend to turn the philosophers discussed into caricatures of themselves. You mention that you are in school; I'd suggest you take a philosophy course or two, if you are interested in the field. Having an experienced guide will definitely help.

The ones I've read and find good: - Existentialism is a Humanism - Jean Paul Sartre : Very easy to read and is a very good introduction to existentialism. - Thus Spoke Zarathustra - Nietzsche - Siddhartha - Herman Hesse