Dark SisterGraham Joyce
British Fantasy Society: Best Novel winner (1993).
Maggie and Alex have their problems. their old, drafty house, for one thing. Their stressed-out marriage, for another.
Then one day, uncovering and cleaning the house's original fireplace, they discover a dead blackbird... and an old handwritten diary full of herbal lore.
Maggie takes an interest in the diary. Soon, with the help of her friends Ash, and herbalist,and Old Liz, and old woman with a deep knowledge of ancient ways, Maggie tried to find her way in a world of power and magic. But Maggie's searching has awakened her Dark Sister, a malevolent force that threatens her hold on her family and her sanity.