The blurb on the back calls this "a masterpiece of fantasy and wonder," and I think it is. I read it long ago and saw fit to keep it, though I have forgotten a lot of the story.
An orphaned ten-year-old girl, living in an enormous, broken-down house with her governess, visits a little island in the middle of a lake and finds it inhabited by tiny people. The adults around her try to keep her from doing what she knows she must do.
This is a children's book, but its involved story, literary and historical references, and good vocabulary make it also enjoyable for adults. My copy is 231 pages of ordinary-sized print--a good-sized read.