One of the most important texts for Rinzai Zen students: Torei Enji's Shumon Mujintoron. This translation, which includes a teisho commentary by Daibi, is an invaluable resource serving not only as a map of the entire Rinzai path, but also as a basic primer of Mahayana and Japanese Buddhism.
It had been published by Tuttle, but eventually went out of print; copies, when found, are around $100. But the Zen Centre in London, UK holds the original copyright and still makes the work available at an affordable price. I ordered this last week and it shipped promptly.
If you'd like a copy, do support the Zen Centre and their good work.
I should mention that they have also kindly allowed me to use quotations from this in my own book...much appreciated!