Introduction to Tantra Shastra by John Woodroffe.
He translated some twenty original Sanskrit texts and, under his pseudonym Arthur Avalon, published and lectured prolifically on Indian philosophy and a wide range of Yoga and Tantra topics..
Alongside his judicial duties he studied Sanskrit and Hindu philosophy and was especially interested in Hindu Tantra.
About the Author Sir John George Woodroffe (1865-1936), also known by his pseudonym Arthur Avalon, was a British Orientalist whose work helped to unleash in the West a deep and wide interest in Hindu philosophy and Yogic practices.
While there are a few caveats, Woodroffe was the first Western writer to treat seriously and sympathetically of the Sakta Tantrik cults and their scriptures.
Originally the extented Introduction to a translation of the Mahanirvana Tantra by "Arthur Avalon" (a collective pseudonym for Woodroffe and an unnamed collaborator), this is a sufficiently general treatment to repay reading apart from the work to which it was originally attached.
Introduction to Tantra Shastra by John Woodroffe