I'm reading Realizing Genjokoan, and the author discusses emptiness, which he describes as the "reality of impermanence" and the " lack of independent existence". These are essential concepts that I would like to explore more, but searching on Amazon, I only find books discussing these concepts from the Tibetan perspective. Any recommendation for a book about emptiness from a Zen point of view? If not, any recommendation for any book on the subject? Thanks.