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.
Kôun Yamada: Gateless Gate
Kôun Yamada: Zen the Authentic Gate
Roselyn Stone: Winding Road Straight Path
Philip Kapleau: The Three Pillars of Zen
Charlotte Joko Beck: Everyday Zen
Shunryu Suzuki: Zen Mind, Beginner’s Mind
John Daido Loori: The Eight Gates of Zen
Taizan Maezumi & Bernie Glassman: On Zen Practice
Joanne Miller: Zen and Gospel of Thomas
Christopher Collingwood: Zen Wisdom for Christians
Susan Murphy: Upside-Down Zen
John Tarrant: Bring Me the Rhinocerous
David Loy: Ecodharma
Subhana Barzaghi: The Red Thread of Zen