The 5 volume set this is part of on Vajrakila by Martin Boord is apparently the definitive English treatise on the subject. I say “apparently” because I’m just now reading this 1st volume (which is a revised and expanded version of his PhD dissertation which was originally published as ‘The Cult of the Deity Vajrakila’ in 1993), and if the other 4 volumes are anything like it, they’re all worth getting!
This volume covers a broad range of subjects from a history of the Northern Treasures School, to the ancient history of the Kila traditions (in archeology, anthropology, mythology), to the iconography, tantras, termas and rituals associated with Vajrakila practice. Boord, as a yogin/scholar who has spent his career focused on this deity and tradition, gives us a brilliant and thorough (fascinating and readable!) authoritative account (throughout he relies on primary source material he’s translated from the Tibetan and Sanskrit, as well as comparative studies of published works in English by other scholars - all documented in copious footnotes - which itself makes for a fascinating list for further reading).
“This image of Vajrakila incorporates traits of both god and demon. As an expression of the human psyche, then, it squarely addresses the problem of what Jung has called the ’shadow’ side of the personality, consisting of those unpleasant aspects that elsewhere may receive only reluctant acknowledgement. The portrait is horrible in an extreme and yet is said to be alluring, magnetic, compulsively attractive. Vajrakila is the embodiment of the absolute truth of the human condition, the unbiased, unflinching presenter of the best and worst in a single icon accessible to the yogin through his meditative training. The demonic is here fully accepted as an aspect of the divine.” (p.157)