Service magicians provided, or at least purported to provide, spells for healing, divination, love, and protection from witchcraft, among other things. In England they were known as “cunning folk”. Their alleged sources of power included birthright, such as being the seventh son of a seventh son (or daughter), encounters with fairies, or possession of grimoires. These correspond, respectively, to D&D’s sorcerer, warlock and wizard classes.
In this excerpt from
Popular Magic: Cunning-Folk in English History (2003), Owen Davies discusses inherited abilities and fairies:
To a certain extent, magical ability was also held to be a natural or inherited gift. This was most evident when it came to healing. Seventh sons and daughters, for example, were believed to possess innate powers to cure certain conditions, and, not surprisingly, cunning-folk often claimed to be so blessed. But such hereditary abilities were usually related to specific fields of practice. They did not imbue the practitioner with comprehensive magical powers. Prior to the eighteenth century some cunning-folk and other healers also claimed to have gained powers from the fairies. In 1438 a Somerset fortune-teller and healer named Agnes Hancock was charged before an ecclesiastical court with communicating with fairies, and claiming that she 'sought their advice whenever she pleased'. The Dorset cunning-man John Walsh told an ecclesiastical court in 1566 that he would go up to the hills where there were 'great heapes of earth' at midday or midnight, and there he would speak with the fairies who would tell him which of his clients were bewitched and where stolen goods could be found. In later decades fairy associations were most likely to be claimed by female practitioners. Joan Willimott, a Leicestershire healer examined for witchcraft in 1618, said she obtained her abilities to help the sick after a man named William Berry 'willed her to open her mouth, and hee would blow into her a Fairy which should doe her good'. In 1645 Ann Jefferies of St Teath, Cornwall, was arrested and questioned about the healing touch she said she had gained from the fairies (p. 70).
Here, the same author gives an account of the use of magic books by cunning folk in his article “Cunning-Folk in England and Wales during the Eighteenth and Nineteenth Centuries” (1997):
Perceptions of inherited knowledge and innate ability (being a seventh son for example) certainly helped generate respect, but so did literacy and 'book learning'. The magical books of cunning-folk were held in great awe, and over and over again it is recounted how cunning-folk impressed their clients by poring over large tomes. The profitable production of written charms also required some degree of literacy, as did the reading and writing of the postal consultations many cunning-folk conducted. The evidence points to the fact that an illiterate cunning-person was unlikely to go very far. As one dissatified farm foreman remarked, after consulting the son of a cunning-woman, in 1889: 'he "was not scholar enuf" to be able to help' (p. 93).