I’m on my way to visit Susan Cooper on an unseasonably warm day in mid-February. As my car cruises along, about 45 minutes south of Boston, low tide reveals miles of untouched marshland. I drive across a short causeway, creep down an unpaved lane, and suddenly I’m staring at the exquisite home that Cooper built a couple of years ago. My first thought is that I’ve stumbled upon the Grey House, the setting of Cooper’s first children’s book, Over Sea, Under Stone. With its soaring cathedral ceilings and wraparound windows that frame the wetlands, the space is filled with warmth and light even on a winter’s day. It seems like the perfect place for the 77-year-old writer to conjure up some more of her magic.