Others have brought up how much Twin Peaks invokes fairytale and mythology, particularly with regards to saints and hermits.
Margaret (The Log Lady) is a seer, a conduit between some other thought or world, translating her log to deliver cryptic but helpful information to our protagonists. At first we thought the wagon wheel on her house was a spinning wheel; it still calls to mind the Norns and other spinners of fate. She knew they would come. They must submit to her rules and rituals ("Wait for the tea!") to obtain access to her knowledge; and they must be polite. Dale lost an earlier opportunity to learn from her because of his lack of faith and respect.
This brings me to Harold, who follows very similar formatting. Donna must come to him, and meet him in his house on his terms. She brings first a physical offering (he meals on wheels) and then a spiritual one: her life story. He then agrees on the condition that he read Laura's journal to her, and that the journal doesn’t leave his home. He makes explicit, even, that she's agreed to a binding arrangement: "A bargain has been struck". He's very fey: bound to his home, surrounded by lurid flowers, and collecting the secrets of those who come to him. I think of Angela Carter's Erl-King, of dryads and other tree spirits, of every fairy deal.
Hell, you can argue that Blackie fits her own template in the same vein; she's the enchantress in her castle, in control of her domain, encorselling the unwary into her service. Audrey (and every other girl) signs a literal contract. Audrey must pass a test, prove she has something worthwhile to offer. And Blackie too is a seer, albeit a self-serving one; she reads the cards every night, divining her future; she transforms her own employees into these same cards- "Pick a card." She even has a little old hunchbacked woman who sews these identities onto the girls, as fitting as any fairytale hag
Of course, the tragic thing about Blackie's control is how arbitrary, even false, it is. She is only the mistress when the owner is away, and dies trying to gain ownership of her own house. She is killed with a kiss, mastered from outside. She invokes Circe, Morgan le Fay, and The Lady of the House of Love, a vampiress and a prisoner.
"Through the darkness of futures past, the magician longs to see."
Both Margaret and Harold see the past; the night on the ridge, Laura's own thoughts. Blackie seeks to see the future. Some give their knowledge willingly, some are tricked, and some never see a way out of their own enchantment.
