Takuan Soho's "The Unfettered Mind" is a good reference. Don't skip the introductory material, though; context matters.
As does practice. Before I came to live here, I was fortunate to be able to practice za-zen for a few years and the teacher was a Buddhist priest, the headmaster of the Japanese school in London. We would sit for 90 minutes, interrupted by the slow walk round the dojo, and then practice aikido--usually sword work, for another 90 minutes. In the UK, the tradition of combining sitting with aikido was started by K Chiba and continued by M Kanetsuka. I never encountered the practice elsewhere, however, least of all here in Japan. Perhaps there is no need, for there are zen temples all over Japan and some of my dojo colleagues would sometimes go off for s few days and stay at a temple. I could never do this because of my university commitments.
My version of Takuan's book is the Kodansha edition with the Scott Wilson translation, the notes of which reveal the problems of making a good translation from a language like Japanese. I am currently reading two books, which are very enlightening: The Invention of Religion in Japan
, by Jason Ananda Josephson, and Sympathy for the Traitor: A Translation Manifesto
, by Mark Polizzotti.