I grew up in Pennsylvania but we moved back to Europe in my early teens and the boxes containing all our children's books got lost in the move.  There is one that I have been pining for ever since!  It was given to me by an old lady who was the grandmother of some people my parents befriended in the States.  She lived in a big house and had an amazing library and I used to love looking at all her old books.  She really sweetly gave this one to me as she could see how much I loved it.  It was a big thick children's book with black and white illustrations and wonderful colour plates and it must have been written somewhere around the early 20th century - I think.  It was about three siblings who are sent to the country to live with I think their grandmother, in a big house.  There are lots of stories about fairies around, and it is clear that the grandmother and the staff have seen them.  The fairies are not always benign.  Their grandmother starts calling the children fairy names like Puck and Robin.  Eventually the children see the fairies too.  There is some disquiet that the youngest child, a little boy, is falling too far under the fairies' spell; for example he falls asleep in the middle of a circle trampled in the grass by the fairies, which apparently means they 'own' him.  It is a very beautiful and sweet book, and I am hoping it might ring bells.  I can't tell you how much I miss it - it has been 40 years since I last saw it.