“Master Yunmen: From the Record of the Chan Master “Gate of the Clouds”” by Urs App includes – as introduction – a (very) brief history of Zen, and a description of the life and of the teaching of Zen Master Yunmen Wenyan (Ummon in Japanese), who lived between 864 and 949 AC.
The influence of Yunmen Wenyan is considerable, because one of the seven branches of Zen is named after him; and in all major koan collections – Gateless Gate, Blue Cliff Record, The Record of Serenity – Yunmen is included more often than other Zen Masters.
Next to the introduction, this book includes 285 talks and dialogues by Yunmen.
As example talk 243 (also included in Koan collections):
Addressing the assembly, Yunmen said:
“I’m not asking you about before the fifteenth day; try to say something about after the fifteenth day!”.
Yunmen himself answered on behalf of the listeners:
“Every day is a good day!”
Summary of the footnote to the fifteenth day: “after the fifteenth day” refers to “completely realising one’s (own) existence”.
Recommended for a further introduction/study of Yunmen.