The essential nature of Zen can be summarized in four short statements:

(1) Special transmission outside the orthodox teaching

(2) Non-dependence on sacred writing

(3) Direct pointing to the human heart

(4) Realize one’s own nature and become a Buddha.

More than any other school, Ch'an stresses the prime importance of the enlightenment experience, which is indefinable, incommunicable, free from all forms and concepts. The authenticity of the enlightenment experience can only be tested by an enlightened.

Ch'an is the immediate expression and actualization of the perfection present, referred to as the identity of Samsara and Nirvana, in every person at every moment. What is important is living truth rather than the useless ritual religious practices and intellectual analysis of doctrine for the attainment of liberation. The general practice of Ch'an was to point directly at one's mind with just a few hint and force one to become enlightened instantly. From this point of view it can be seen that the Ch'an practice depends on no words and emphasizes no setting up of words and letters.

Zen also teaches the practice of zazen, sitting in meditative absorption as the shortest but also the steepest way to awakening.

Zen's Chinese Heritage -  A book review by Andrew May
Zen Ancestors  -  The Masters & Their Teachings
Introduction to Zen Buddhism  about.com
Ch'an Masters: Patriarchs and other great Zen masters of ancient China   - Teachings of the great masters of sudden illumination in ancient China -- the forerunners of Zen in Japan
WOMEN IN ZEN BUDDHISM: Chinese Bhiksunis in the Ch'an Tradition  -  Bhiksunis and Nuns in the History of Zen
About Zen - http://zen.thetao.info/
The Complete Idiot's Guide to Zen Living by Gary McClain, Eve Adamson
Discover Zen: A Practical Guide to Personal Serenity by David Fontana
Everyday Zen: Love and Work by Charlotte J. Beck
An Introduction to Zen Buddhism by D.T. Suzuki
Manual of Zen Buddhism by Daisetz Teitaro Suzuki
The Method of Zen by Eugen Herrigel
Not Always So: Practicing the True Spirit of Zen by Shunryu Suzuki, Edward Espe Brown
Nothing Special: Living Zen by Charlotte J. Beck
The Pocket Zen Reader by Thomas Clery
Sanctuary: Music from a Zen Garden - Riley Lee - audio CD
The Three Pillars of Zen: Teaching, Practice, and Enlightenment by Phillip Kapleau Roshi - a book 
Traditions and Schools of Buddhism - http://buddhismcanada.com/schools.html
Zen Action/Zen Person by T.P. Kasulis
Zen and the Brain: Toward an Understanding of Meditation and Consciousness by James H. Austin
The Zen Commandments: Ten Suggestions for a Life of Inner Freedom by Dean Sluyter
Zen Flesh, Zen Bones by Paul Reps
Zen Meditation in Plain English by John Daishin Buksbazen, Peter Matthiessen
Zen Mind, Beginner's Mind by Shunryu Suzuki
Zen Miracles: Finding Peace in an Insane World by Brenda Shoshanna
Zen Schools in Japan - http://terebess.hu/english/lexikon/zen.html
Zen Studies Society - http://www.daibosatsu.org/
Zen: The Path of Paradox by Osho
Zen 24/7: All Zen, All the Time by Philip T. Sudo
Zen Training: Methods and Philosophy by Katsuki Sekida