The book is anecdotal, capturing a series of dialogues that Huang Po has with students. Depending upon the student he is addressing, these range in topic, but all inevitably focus upon one theme--Awakening.
Huang Po has one teaching: Find your true nature, One Mind, the true non-dual substance of the universe. The way to do that? Cut of all discriminating thought. He writes:
Only when your minds cease dwelling upon anything whatsoever will you come to an understanding of the true way of Zen...[T]he way of the Buddhas flourishes in a mind utterly freed from conceptual thought processes.We live in a world of concepts. Self, other, me, mine, yours, up down, inside, outside--these are all ideas, ones that Zen Master Huang Po obliterates with one swipe of Manjushri's wisdom sword. Time and again he reminds us that neither merit nor scholarship can lead us to the One Mind. It is our true nature--indeed the nature of the universe--so we don't need to strive too hard to find it. In fact, the more we strive, the farther away Nirvana goes because we are separating ourselves from in, when in reality we are it.
Huang Po accepts nothing more than complete and utter non-duality. According to Zen, reality is one seamless, undifferentiated whole, in terms of time, space, cause, effect, thought, matter, and so on. It excludes nothing. Anything less is incomplete, or an idea masquerading as reality.
I'd feel like I was misrepresenting Huang Po if I didn't mention that he was also a proponent of sudden enlightenment, a school of thought that argued that enlightenment does not occur in stages (for that would mean someone can be partially enlightened) but rather all at once. Apparently this was a big deal back in the T'ang Dynasty (around the 800s) in China, but not so much now. I think people today would be happy to take any kind of enlightenment they can--sudden, slow, complete, incomplete, diet, regular.
If you haven't already, put The Zen Teaching of Huang Po on your short reading list. It's an invaluable addition to any Zen library.