Baraka_Guru

I'll suggest three good books I've read that have strong existentialist themes:

The Stranger, Albert Camus
Steppenwolf, Herman Hesse
"Waiting for Godot," Samuel Beckett

Your biggest challenge should be to clearly distinguish between existentialism, nihilism, and absurdism. Though related, they are distinct parts. Beckett's play, for example, is absurdist more than existentialist proper. Also consider Camus' Myth of Sisyphus in that respect.

Unfortunately, I didn't actually study existentialism, though it is an interest of mine. I read those works with the philosophy in mind. I think asaris' suggestions are on the mark if you want to go to the core texts. Also consider reading critical works, as core philosophy texts can muddle the mind.

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Knowing that death is certain and that the time of death is uncertain, what's the most important thing?
—Bhikkhuni Pema Chödrön

Humankind cannot bear very much reality.
—From "Burnt Norton," Four Quartets (1936), T. S. Eliot