Published in 1988, Wicca: A Guide for the Solitary Practitioner set out an explicit ritual framework, seasonal calendar and ethical code that a reader could adopt alone, without initiation by an existing priest or priestess and without access to any coven's closely guarded Book of Shadows. That was a genuine departure from the initiatory model of Gardnerian and Alexandrian Wicca, in which lineage and secrecy were treated as essential to legitimate practice, and traditionalists at the time argued that Cunningham was giving away, in paperback, what earlier generations had guarded as oath bound material.
The book's effect was to help move American Wicca decisively toward the eclectic, self initiated form that is now, by most estimates, more common in North America than any coven based lineage, a shift scholars of contemporary Paganism generally credit as much to Cunningham's accessible solitary format as to any single other cause. His companion volumes, the Encyclopedia of Magical Herbs and the Encyclopedia of Crystal, Gem and Metal Magic, performed a similar service for folk magical correspondence, becoming standard practical references cited across the wider Pagan and New Age publishing world well beyond Wicca proper.