I don't know how old you are (the person who started this thread), but when I was 17-18 the books that had a most profound influence on me were that of Hesse, Camus and Nietzche. To name a few:
Hesse, Sidhartta.
Hesse, Demian.
Hesse, Steppen Wolf
Camus, The stranger
Camus, The myth of...