The author of the Zazen Manual is Ch’ang-lu Tsung-tse, a monk in the Chan (Zen) and Pure Land traditions in 11th century China. However, the details are unclear and the work is only attributed to him. (For reference, see Carl Bielefeldt’s Dôgen’s Manuals of Zen Meditation where it is shown that much of Dôgen’s classic work on the subject is based on this earlier work). This is also the Zazen Manual used by Rinzai monks to this day, and included in the classic Four Texts of the Zen School and the two-volume Poison-Painted Drum, the handbook for Rinzai monastic practice.
(This Zazen Manual was also the subject of Jeff’s 2015 Spring retreat lectures. A transcribed summary of these lectures can be found here: https://beingwithoutself.files.wordpress.com/2015/05/spring-2015-retreat-lectures.pdf ).