Zen Mind, Beginner's Mind by Shunryu Suzuki is a wonderfully insightful book into Zen practice. Do not be fooled by the title, "...Beginner's Mind," for it's contents are for the experienced practitioner as well.
The book was originally published forty-one years ago, and to this day is one of the essentials to any Zen library. It's written from recordings of Suzuki Roshi's talks he gave to his students. The grammatical style reflects this by rarely editing the structure of how he spoke, which gives you the feeling that Suzuki is talking right to you. In fact, Shunryu Suzuki himself said, "Nice book, but I didn't write it." It was actually edited by one of his long time students Trudy Dixon.
Zen Mind, Beginner's Mind talks about the essence of Zazen practice, the Zen mind, and other very insightful Zen/Buddhist matters. I'm definitely not getting rid of this book, as I foresee myself referencing back to it many times in my practice.
I hope you enjoy this book as much as I and many other Western practitioners have.
Suzuki Roshi (1904-1971) was a Soto Zen priest from Japan who established the popular San Fransisco Zen Center in 1959, and the Tassajara Zen Mountain Center. He stayed in America until his death, and is seen as one of the most influential people to bring Zen and Buddhism to the West.