Merton felt that his journals contained his best writing. I'll offer a different opinion; I think his essays and book reviews contain much of his best writing. "Zen and the Birds of Appetite" is a collection of essays on what's common to Zen and Christianity, and the book includes a book review and Merton's prefaces to two books by other authors.
He seems to write these prefaces not simply because he was asked to. He writes them, I think, because the books really inspire him. (Most of us write these reviews on Amazon.com for the same reason!) His prefaces present his thinking along with the author's thinking in a way that improves the overall publication. Comparing his thinking with another author's thinking seems to make Merton's writing even more succinct and sharply-reasoned than usual. And in "Zen" he's comparing his faith with another faith, so his sensitivity, appreciation, and sharp mind are even more in evidence than usual.
These essays don't amount to a textbook on Zen or Zen Buddhism, any more than a collection of short stories adds up to a novel. But together the essays address an overall question: what is it about Christianity that resembles Zen? In the process of approaching the question, Merton gives us some gems. His discussion of paradise, innocence, and knowledge is the best I've read. You may learn more about Christianity than about Zen in this volume.
His essays make up the first part of the book. The second part of the book is a "dialogue" between Merton and Diasetz T. Suzuki, a Zen scholar quite accessable to the Western mindset. These dialogue seems to devolve somewhat into a "point-counterpoint" duel, but that's fun and a lot of well-framed truth comes out.
Merton quotes D.T. Suzuki: "Zen always aims at grasping the central fact of life, which can never be brought to the dissecting table of the intellect"; and "Zen must be seized with bare hands, with no gloves on." No wonder Merton's reverence for Zen, for these are his own ideas of Christian monasticism. With his illuminating mind in full stride, and his interventions keen as crystal, if he went no deeper than to make an apparent synthesis, it would be enough. But Merton strives for farther fields, finds and feeds them, and not surprisingly leaves them flourishing. He leaps wholly into a personal embodiment of Zen and its spiritual complexities, and ends restoring his own monastic experience. The essay 'Zen in Japanese Art' pays loving homage to the classic spirit of Daisetz Suzuki's seminal 'Zen and Japanese Culture', but lives and breathes on its own. In its simple three and a half pages, Merton weaves the aesthetic ideas of Zen philosopher Kitaro Nishida, makes the case that Zen and Zen art are the exact opposite of Sartre's 'pessimistic nihilism,' and in a single amazing paragraph toward the end, beautifully finds in the formal "tea ceremony" a respect and harmony consistent with the simplicity of twelfth-century Cistercian architecture at Fontenay or Le Thoronet. But no idle intellectual excursions invade here; again and again Merton draws everything back to the Christ sought in the apophatic tradition with a faithfulness that exhudes an almost excruciating surfeit of spiritual understanding. Finding St Gregory's "No one gets so much of God as the man who is thoroughly dead" 'lying next' to Bunan Zenji's "While alive, be dead, thoroughly dead-- All is good then, whatever you may do", Merton turns a light on centuries of Christian ascetic experience with one true, bold stroke. Birds of Appetite is strewn with page after page not of ideas only, but wisdom. He responds to D.T. Suzuki's exquisite essay 'Innocence and Knowledge' (included in the book) with 'The Recovery of Paradise', arguing that the Desert Fathers sought the emptiness and innocence of Adam and Eve in Eden, invoking along the way John of the Cross, and making one of Dostoevsky's "saints," the Staretz Zosima, serve as antagonist throughout the essay. Merton notes "there is a dimension where the bottom drops out of the world of factuality and of the ordinary," an observation no doubt honed in the solitude of the hermitage, up the mountain above Gethsemane Abbey. He adds, "it might be good to open our eyes and see." I'm recommending a huge little book, meticulously published by New Directions with its customary attentiveness to shadow and light, inside and out. See for yourself.