Zen underwent huge development and change in Japan after it was introduced in the 12th century, notably the division in the Soto and Rinzai schools (originally the Caodong and Linji schools in China). Kwan Um School students don’t necessarily need to delve deeply into Japanese Zen, but it’s important to be familiar with a few of the most important figures—notably Dogen and Hakuin. 

Kazuaki Tanahashi, Moon in a Dewdrop: Selected Writings of Dogen: An excellent introduction to the voluminous and inescapably difficult works of Dogen.

Norman Waddell, The Essential Teachings of Zen Master Hakuin: A short selection of Hakuin Zenji’s clear and accessible writings, including his essential “Song of Zazen.” 

Shunryu Suzuki Roshi, Zen Mind, Beginner’s Mind: The classic teachings on Soto Zen presented by the founder of the San Francisco Zen Center—one of the best books to give an absolute beginner.