There are many excellent (and as many not-so-excellent) books on Zen practice in your local libraries and bookstores. They can be invaluable guides, but never forget that they are only guides, not the practice itself. A book about sailing may be filled with information that will be useful once you're out on the water, but sitting at home reading is not the same as setting out in your boat.
Nothing that you read (this instruction manual included) becomes true until you discover the truth or falsehood of it for yourself in your own practice. While books can offer much useful advice and encouragement, please remember that reading a book is not a substitute for sitting. You cannot practice Zen vicariously.
Some of the better books that you may want to read are:
by Robert Aitken Roshi
A concise introduction to zazen.
by Shunryu Suzuki Roshi
A collection of talks about Zen practice.
by Philip Kapleau Roshi
One of the most famous books about Zen in English.
by Katsuki Sekida
An excellent, detailed guidebook on how to approach Zen practice.
by Will Johnson
A slim volume with a world of thoughts about how to sit and the effects and meaning of posture.