There are many books written about Zen. As with any subject there are huge variations in depth and quality of writing. The following books are recommended for beginners and experienced practitioners alike.
Philip Kapleau The Three Pillars of Zen
Yoko Beck Everyday Zen
Shunryu Suzuki Zen Mind, Beginner’s Mind
Kôun Yamada Gateless Gate
John Daido Loori The Eight Gates of Zen
Roselyn Stone Winding Road Straight Path