This is one of Trungpa Rinpoche's early teachings in 1982 in which he introduced basic tantra to Western audiences. During this time t Vajrayana practice was little known outside of a few teachers such as Kalu Rinpoche in the US. 

This book discusses Vajra Nature, the Mandala, the purpose of empowerments, how to visualize and four tantras. These discussions are very down to earth and are explained in a language that understood modern culture. This is why this book is still relevant to our time.