At about 160 pages, The Book: On the Taboo Against Knowing Who You Are (1966) by Alan Watts is a fairly short read but leaves you with plenty to ponder. In it he touches on a broad range of doctrines and beliefs, from pantheism and panentheism to Hinduism and Chinese philosophy, while presenting a worldview that everything, the entire universe, is one, and that we—each as a self-perceived "skin-encapsulated ego"—are not in any way separate entities from this universe.
I liked The Book for how Watts can get a Western mind looking at things—at itself, at everything it has learned and thinks it knows—with something of an Eastern perspective. He incorporates science into the philosophy as well, asserting near the beginning that all living beings have always been in essence tubes that put food in one end and let it out the other, which at some point in our evolution grow tools like nerves, brains, eyes and ears to better scrounge up things to swallow. I'd put off reading The Book for years, imagining it might be a bit too hippie for my taste. But apart from some mention of the use of consciousness-changing chemicals and a handful of other particularly 1960s-ish attitudes, his ideas feel fresh, or timeless in their universality. What struck me as especially relevant to the present day, and which contemplative environmentalists might agree with, is Watts' idea that our individual sensation of having a separate ego—rather than a sensation of mind and body as one, or of being one with everything else—underlies, as Watts put it, "the violent subjugation of man’s natural environment and, consequently, its eventual destruction.”
This is the first Watts book I've read. I found it unexpectedly accessible, and considering how he lays out his ideas on human (traditionally Western) notions of duality and their effects on the mind, I figured it's a good place to start before taking on his books related to or specifically about Zen.