Some good reads:

"Essays in Existentialism", "Being & Nothingness", "Existentialism & Humanism", "The Wall", "Nausea" by Jean-Paul Sartre

"The Myth of Sisyphus", "Resistance, Rebellion & Death", "The Rebel", "The Plague", "The Stranger", "A Happy Death", "Exile & The Kingdom" by Albert Camus

"Being & Time" by Martin Heidegger

"The Existentialist's Guide to Death, the Universe, & Nothingness", "How to Be an Existentialist: Or How to Get Real, Get a Grip, and Stop Making Excuses", "Sartre: A Guide for the Perplexed" by Gary Cox

"Thus Spake Zarathustra", "Beyond Good & Evil", "Will to Power", "The Gay Science", "The Antichrist", "Ecce Homo", "The Geneology of Morals", "Twilight of the Idols" by Friedrich Nietzche

"Existentialism: A Guide for the Perplexed" by Steven Earshaw

"Introducing Existentialism" by Richard Appignanesi

"Man's Search for Meaning", "The Will to Meaning" by Viktor Frankl

"The Sickness Unto Death", "The Concept of Anxiety", "The Concept of Dread", "Works of Love", "Fear & Trembling", by Soren Kierkegaard.