It doesn't seem there are really any bad Thich Nhat Hanh books. Somehow the deep practical wisdom you can feel in them keeps his simple writing style from toppling over into self-help platitudes. I think anyone, regardless of whether they are a buddhist or a meditator can profit from reading things like Anger or Being Peace. But while The Heart of the Buddha's Teaching has plenty of practical advice for better everyday living, it's a little different from those others. Here, Hanh elaborates his own understanding of the classic concepts of Buddhist doctrine. There are many chapters devoted to the four noble truths, one for each limb of the eightfold path, and others for the various lists that Buddhists seem so drawn to, such as the three dharma seals (aka characteristics) the four immeasurable minds (brahmaviharas), the five aggregates, the seven factors of awakening, etc ... In other words, this is not really a book for beginners. It's clearly aimed at people who already practice, who are already familiar with some of these sanskrit terms and ideas, and who want to get a fuller and more precise sense of what the Buddha had to teach.
Which is not to say that Hanh just presents a straight-up traditional account of Buddhism. He wants to make these ideas accessible to a modern audience, and he proposes making some important changes in our understanding of the traditional formulations in order to do this. His biggest beef is with the traditional focus on the centrality of suffering. Indeed, he goes so far as to say that dukkha is not really even the third characteristic, at least if we consider these are the ultimate descriptors of what is. Suffering is not universal in the same way as impermanence and non-self. Instead, Hanh replaces dukkha with nirvana in his formulation. Nirvana is actually the third fundamental and universal characteristic of everything, which is then obscured by suffering. We didn't need the Buddha didn't teach us that there is suffering or that life is unsatisfactory -- we needed him to teach us that there is more than this, that there is a way beyond suffering and struggle. It's a substantial and, I think, wonderful substitution that changes our whole perspective on the tradition. Instead of reading the three characteristics as negations (im-permanence, non-self) it allows us to read them as positive and liberating factors. Nothing lasts forever. We are not separated from an alien universe. And if we look into, accept, and let go of our suffering, and our clinging to permanece, self, and suffering, we can find the unimpeded joy that's already present. Awakening, in other words, is a positive process of unblocking what's already in this world, not a negative one of leaving it behind. We only negate the negations, the things that hold us back.
Hanh's shift towards a more positive interpretation of Buddhism fits well with another (for me) surprising aspect of the book-- his heavy emphasis on the non-dual. Though he doesn't mention the concept much by name, he speaks repeatedly of transcending the subject and object, existence and non-existence, and is constantly reminding us of the "interbeing" of all things. What I'm calling his non-dual subtext reaches a climax in the penultimate chapter on dependent origination. Hanh would like to see the model of 12 links arranged in a linear causal chain, replaced with 10 links arranged in a network where each is connected to all the others. He calls this "interdependent co-arising" and uses one of the Buddha's images of three reeds standing up by leaning on one another, teepee style, to illustrate how everything co-is, so to speak. This network or vortex model of causality is dear to my heart, so I enjoyed the overall aim of the presentation even if the particular 10 or 12 links involved seem kinda arbitrary to me. I would be interested to know what other people with more commitment to the traditional scheme think of his re-working.