I love The Blue Castle! Read it earlier this year for the first time and it was an instant favorite. Maybe a bit of a soap opera with all the convenient twists and coincidences, but I just love it. The characterization is great and the nature writing is beautiful.

Not to turn this into a recommendation thread, but you might also like Rebecca by Daphne du Maurier, The Haunting of Hill House by Shirley Jackson, and Jane Eyre and Villette by Charlotte Brontë, which have similar main characters and also really gorgeous prose.