I remember reading this book in 1977, so it predates that year. There's a young girl who finds an abandoned mansion with an eerie gothic garden. I recall that the setting is the American Eastern Seaboard/New England area. She feels compelled to explore the house, and by entering and reading a book in the dusty library she is transported to another world. There's a lot of scene focus on the ocean and coastal settings, really evocative of the wind and the salt spray in a similar way that McCaffrey lovingly wrote the first few chapters of Dragonsong. The protagonist is not a rebel or an outcast escaping our mundane world, just a normal schoolgirl who inexplicably feels more at home in her new setting. She goes on adventures and proves herself through many trials in classic YA isekai/fantasy style. I was very young when I read this, and I would like to find it again to see if it's as good as I remember.