"Sent away from her foster home one long, hot summer to a sleepy Norfolk village by the sea, Anna dreams her days among the sand-hills and marshes. She never expected to meet a friend like Marnie, someone who doesn't judge Anna for been ordinary and not-even-trying. But no sooner as Anna learned the loveliness of friendship, then Marnie vanishes..."
This is the book that inspired Yonebayashi Hiromasa, from Ghibli Studio in his 2014 piece Omoide no Maanii (????????).