There are two ways to approach the teachings on emptiness: one can study the vast Prajna-paramita sutras and commentaries using logic until one has grasped this intellectually or one can look directly at mind in one's meditation.

The Harmony of Views is a fascination book which contains three full-length commentaries by Thrangu Rinpoche on the nature of mind from three different Tibetan traditions. He takes three different spiritual songs (dohas), one from the Geluga master Changka Dolpay Dorje, one from the great Nyingma master Ji Mipham, and one from the well-known Kagyu master Chogyam Trungpa and shows how these are harmoneous with one another.

Available also a paperback for $ 13.00 in the Book section of this website.

You may also view the complete teachings in this book on YouTube for free. The link is: 
youtube.com/NamoBuddhaPublications.