There is so much excellent information about the history of Buddhism and Dao in China, which Xu Yun rose up thru, and straight information on the practice of Hua Tou which is the practice which he taught.
imho, hua tou is the clearest practice that the western mind can fathom.
https://wenshuchan-online.weebly.com/biography-master-xu-yun-1840-1959.html