Where to start?
There is my own Existentialism: A Guide for the Perplexed (Continuum, 2006 - copies in the library if you don't want to shell out, though it's clearly a brilliant book and one that you will want to keep). Other introductions - and it's definitely worth reading others for quite different perspectives - are John Macquarrie's Existentialism and David E. Cooper's Existentialism: A Reconstruction (Second Edition 1999). The Existentialist Reader: An Anthology of Key Texts (ed. Paul S. MacDonald 2000, Edinburgh University Press) also has a very good introductory chapter. Sartre's Existentialism and Humanism is another good place to start. Other recent introductions - and there are now quite a few: Thomas R. Flynn, Existentialism: A Very Short Introduction (2006); Thomas E. Wartenberg, Existentialism: A Beginner's Guide (2008) ; Christopher Pranz, Existentialism for Dummies (2008); Gary Cox, How to Be an Existentialist (2011); Kevin Aho, Existentialism: An Introduction (2014).

Barrett, William. Irrational Man: A Study in Existential Philosophy.

Bedford, M. Existentialism and Creativity.

Blackham, H. J. Six Existentialist Thinkers.

Buber, Martin. I and Thou.

Charlesworth, Max. The Existentialists and Jean-Paul Sartre.

Cooper, David E., Existentialism: A Reconstruction.

Craib, Ian. Existentialism and Sociology: A Study of Jean-Paul Sartre.

Earnshaw, Steven. Existentialism: A Guide for the Perplexed.

Foulquie, Paul. Existentialism.

Goffman, E. The Presentation of Self in Everyday Life.

Grossman, Reinhardt. Phenomenology and Existentialism.

Heidegger, Martin. Being and Time.

Jaspers, Karl. The Philosophy of Existence.

Kearney, Richard. The Wake of Imagination. (Includes chapter on 'The Existential Imagination').

Kern, Edith. Existential Thought and Fictional Technique.

Kierkegaard, Søren. Fear and Trembling.

Laing, R. D. The Divided Self.

Lehan, R. A Dangerous Crossing.

MacDonald, Paul S. (ed)., The Existentialist Reader.

Macquarrie, John. Existentialism.

Marcel, Gabriel. Being and Having.

Pike, Shirley. Marxism and Phenomenology.

Roubiczek, Paul. Existentialism.

Sartre, Jean-Paul. Being and Nothingness.

---. Existentialism and Humanism.

Schroeder, W R. Sartre and his Predecessors.

Solomon, R. C. From Rationalism to Existentialism.

Spiegelberg, H. The Phenomenological Movement.

Wahl, Jean. Philosophies of Existence.

Warnock, Mary. Existentialism.

---. Existentialist Ethics.