Está longe de ser uma combinação perfeita, mas poderia ser o The Infinity Concerto de Greg Bear (publicado em 1984)? O protagonista começa no mundo moderno e entra em um mundo mágico (embora não pense em pular uma cerca, não acho), e é ensinado a magia por um trio de velhas bruxas, que requer alguns métodos bastante duros.
Do resumo da Wikipedia:
The book's present story follows the experiences of a young man named Michael Perrin, a would be poet ... Following the instructions on a piece of paper he found inserted in the book, Perrin enters Clarkham's house, which has been vacant for decades, and soon finds himself in Sidhedark, a world inhabited by a powerful race of beings calling themselves the Sidhe ... She orders him to go see the Crane Women, who are a trio of "Breeds" (part human, part Sidhe), who live on the outskirts of another town populated by other Breeds called Halftown. They train him with exercises that seem pointless and incomprehensible.
Aqui está a capa do livro que eu conheço:
