Here is Sartre's major work "Being and Nothingness." I've only read about half of this book and, to be honest, I find that the author borrows too much from Heidegger and has not developed his own ideas as fully as I would have liked. He is hard to understand at times and sometimes he doesn't make much sense. However, much of this book is thought provoking and it is "the key text of Existentialism." It's a must read for those interested in 20th century Continental philosophy.