Pagans who wish to pursue a fully professional credential can pursue ordination through the Unitarian Universalist (UU) Association, since the UU community accepts Pagans both as members and as clergypersons. However, ordination within the UU community leads specifically to credentials as a UU minister, which means that one's identity as a Pagan becomes a descriptive quality, rather than the central defining feature, of the individual's identity as a clergyperson.
Within most groups, however, issues such as credentials or the ability to minister to non-group members is not an issue, and most Pagan clergy exercise their ministry in collaborative community with others who function as their peers, sometimes under the tutelage of one or more elders who function as mentors. This relatively informal and unstructured approach to ministry works because it diffuses power and authority among the group membership, which minimizes problems such as abuse or egotism and also enables each clergyperson to work without pay, since the responsibilities of the priesthood are shared with others.
Books You Might Enjoy:
Raymond Buckland - Bucklands Book For Spirit Communications
Lyall Watson - Supernature
Anonymous - Confessio Fraternitatis
Sepharial - A Manual Of Occultism