This is a very concise book (originally entitled "The Unchanging Convergence") of pith instructions from a great master, addressing Mahamudra, Dzogchen, and Madhyamaka. I also recommend his "Lamp of Mahamudra" and "Mirror of Mindfulness." In the Introduction to this book, Chökyi Nyima Rinpoche says it "contains extremely precious and crucial advice." His father, Tulku Urgyen, wrote the somewhat similar "Vajra Speech" which summarizes his work; it also has great breadth & depth in a very few pages. While many of the points made in "The Heart of the Matter" are not new, this short book can be referred to and re-read again and again during one's practice to quickly and easily remind us of our ground, path, and fruit. The author quotes numerous sources and makes some very interesting observations:
p. 60: quoting Shavaripa: "emptiness cannot imagine emptiness."
p. 61: quoting Shavaripa: "The supreme meditation is to never depart from the nature of nonmeditation."
p. 71: "The meditator and the meditation object are the mind itself."
p. 73: "Though named `realization,' it is uncorrupted awareness itself."
p. 85: "The Sutra of the king of Samadhi describes this: `Whichever sutra I have taught in all the world systems, the words have but a single meaning. You cannot practice all of them, but by practicing just a single sentence, you will be practicing them all."
p. 98-9: "Decide that whatever you experience is the playful expression of awareness...Since all appearance and existence is the magical display of this single expanse of awareness, the `ultimate view' is to see your mind in utterly naked naturalness. `Meditation training' is to remain in this continuously. `Ensuing cognition' is when a thought is projected. `Post-meditation' is to recognize that projection. `Conduct' is to mingle walking, sitting, and all other activities with the state of awareness."
He emphasizes experience over study-and specifically exhorts women (p. 96) to actively pursue enlightenment. Since I just completed Tsultrim Allione's "Women of Wisdom," this struck a note.