Reddit mentions of Essays in Zen Buddhism, First Series

Who Rules the World? – Noam Chomsky

Siddhartha– Herman Hesse

The Way of Zen– Alan Watts

Dropping Ashes on the Buddha

The Platform Sutra of Huineng

Dogen on Meditation and Thinking

Essays in Zen Buddhism by DT Suzuki

Lyrical and Critical Essays- Albert Camus

The Myth of Sisyphus- Albert Camus

The Metamorphosis- Franz Kafka

The Stranger- Albert Camus

The Rebel- Albert Camus

Man's Search for Meaning- Viktor Frankl

On The Advantages and Disadvantages of History for Life- Nietzsche

Frankenstein– Mary Shelley (Surprising, but highly existential)

Nausea– Jean-Paul Sartre

The Awakening- Kate Chopin

Candide- Voltaire

The Gateless Gate (Mumonkan)

Couldn't get all the PDF's at the moment, but just google the name of the book and then "PDF" for a potential online copy. Highly recommend all of these– Frankenstein and The Myth of Sisyphus are must in general. Alan Watts is great for synthesizing Zen thought as well.

This chakra balancing meditation is great

Im reading this translation and Philosophy of the yoga sutras right now and its really in depth and good

Nietzsche has many profound insights on human nature.
http://nietzsche.holtof.com/reader/index.html

This book is amazing and the chapter "Enlightenment and Ignorance" is such a detailed and clear explanation of the Buddhas idea of enlightenment and his take on what emptiness has to do with our being and existence. The Buddha argues against an only meditative resignation mindset. He think that you should seek the truth and be at one with reality and that the ignorance will blind you.
https://www.amazon.com/Essays-Buddhism-First-D-T-Suzuki/dp/0802151183

Francis Bacon on Society and human nature


Ethics of Ambiguity by Simone de Beauviour

I would reccomend Kierkegaard, Foucault, Marx, Hannah Arendt works on human nature, society, existence.

This is another good translation

http://www.swamij.com/yoga-sutras-list.htm

http://www.contrib.andrew.cmu.edu/~randall/Readings%20W2/Horkheimer_Max_Adorno_Theodor_W_Dialectic_of_Enlightenment_Philosophical_Fragments.pdf

http://www.amazon.com/Essays-Zen-Buddhism-First-Series/dp/0802151183

This one is greatly recommended by Alan Watts :) You should also check out his books, the way of zen and other books on meditation and zen! His work is really entertaining and eye opening.