Essence of Tantra by His Holiness the Dalai Lama explains how tantra is a highly practical and compassionate system of spiritual development. In it, His Holiness discusses refuge, the three paths, greatness of mantra, clear light and initiation. 

Tsong-ka-pa presents the first part of the Great Exposition of Secret Mantra, his seminal text on the paths to buddhahood, vajra vehicle, deity yoga, and method in the four tantras. 

Jeffrey Hopkins finishes with a discussion of the meaning of emptiness, transformation, and the purpose of the four tantras.