An initiation is a part of the practice of tantra, which Lama Yeshe describes in the free book, Essence of Tibetan Buddhism. In an initiation, a qualified Vajrayana teacher empowers a student to undertake certain practices. In order to read books describing such practices, one needs to have received initiation into the particular tantric deity concerned.
In the case of Lama Zopa Rinpoche's Teachings about Heruka and Yamantaka, Rinpoche has requested that they be provided to initiates into those specific practices only.
For more detailed information, see Lama Yeshe's book, Introduction to Tantra, Wisdom Publications, 1987. Thank you so much for your interest in this profound path.
If you would like any of these books or commentaries, please complete the Initiates Only Materials Request Form.