Voss said:
Probably the Malleus Maleficarum, pretty much the first defining text on witches, shaping the entire concept and debate from the 15th century on. (and all the books on witchcraft and demonology that came after it). Witches with a devil pact are largely the only interpretation with a historical basis until the 19th c.
Well, yes, sort of, but the Malleus Maleficarum was a work of propaganda and fearmongering that was clearly written to be a tool in destroying heterodoxy and shore up the authority of the Pope. Before the Malleus and the Inquisition, the holidays and traditions that were a remnant of European Paganism were generally considered to be harmless heathen nonsense, not devil worship. Yes, huge generalization, but c'mon, it's a gaming forum.
Most importantly, it wasn't a work of anthropology. There are reports by Roman historians and early church scholars that actually described the rites of druids and other pagan priests/worshipers which are more historically useful. Just like you shouldn't use the Protocols of the Elders of Zion as a work of scholarly study on Judaism, you shouldn't read the Malleus and consider yourself familiar with what real Medieval European pagans actually did.
All that said, I absolutely agree that the word "witch" has been dominated by connotations of devil-worship in the past few centuries, and the Malleus and Inquisition (and Puritans a century and change later) are the reason.
As for the OP's question, I agree that it all depends on one's concept of witchcraft. Mine, for instance, would be a rural and rustic "wise woman" trained in a mistress-apprentice tradition with a pragmatic mixture of supernatural tricks. Primal magic, psionics, arcane rituals, herbal medicines and poisons, maybe even divine magic from moon-worship or following the green goddess and horned god. As for the classes available in the PHB, seems like fey-pact warlock mixed with cleric might be the way to go, or just make your own.
Also, yay Granny Weatherwax. And Tiffany Aching.