Bojo Jinul's seminal work, the Secrets on Cultivating the Mind (Susimkyeol), contains the essential teachings of Zen that everyone eager to liberate all beings should study carefully. It answers many questions that people who learn about Zen find difficult to answer. It shows the actual nature of mind and advises on how to go on from the initial enlightenment.
Cleary's translation (PDF)
"Since all dharmas are like dreams or conjuring tricks, deluded thoughts are originally calm and the dusty sense-spheres are originally empty. At the point where all dharmas are empty, the numinous awareness is unobscured. That is, this mind of void and calm, numinous awareness is your original face. It is also the dharma-seal transmitted without a break by all the buddhas of the three time-periods, the successive generations of patriarchs and teachers, and the spiritual advisors of this world. If you awaken to this mind, then this is truly what is called not climbing the rungs of a ladder: you ascend straight to the stage of buddhahood and each step transcends the three realms of existence. Returning home, your doubts will be instantly resolved, and you will become the teacher of humans and divinities. Endowed with both compassion and wisdom and fully endowed with the twofold benefit, you will be worthy of receiving the offerings of humans and divinities. Each day you can use ten-thousand taels of gold. If you can do this, you will be a great man who will have indeed finished the tasks of this life."
(Moguja’s Secrets on Cultivating the Mind in Collected Works of Korean Buddhism, Vol. 2, p. 219)