Dōgen called zazen "the front gate for buddha-dharma."  Hakuin called zazen "the gateway to freedom."  No book can replace the practice of zazen, but it's sometimes helpful to find our bearings and derive inspiration from the words of others. 

Buddhist Basics
        Rupert Gethin, The Foundations of Buddhism

The Pali Canon
        Bhikkhu Nanamoli & Bhikkhu Bodhi, The Middle Length Discourses of the Buddha
        Maurice Walshe, The Long Discourses of the Buddha
        Bhikkhu Bodhi, The Connected Discourses of the Buddha

Our Practice Tradition
        Philip Kapleau, The Three Pillars of Zen
        Philip Kapleau, Awakening to Zen

Zen Sources
        Andrew Ferguson, Zen's Chinese Heritage
        Red Pine, The Zen Teaching of Bodhidharma
        Burton Watson, The Zen Teachings of Master Lin-Chi
        Kazuaki Tanahashi, ed., Moon in a Dewdrop: Writings of Zen Master Dōgen