The Children of Green Knowe by L. M. Boston
Mrs. Oldknow, what a cool grandmother. She lets Tolly bring birds in the house because of the freezing weather outside. Actually, she is Tolly great-grandmother and Tolly comes to live with her because his father and step-mother are in Burma. The great-grandmother and Tolly have never met prior to this point. They are such kindred spirits that she inquires as to what name he would like to be called. The great-grandmother lives in an old ancestral manor in the countryside of England.
Mrs. Oldknowe doesn't immediately divulge the entire secrets of this magic house. Tolly soon discovers three magic children that lived there in the past. One brother is Alexander, who plays the flute. The other brother is Toby, who rides the magical horse named Feste. The spunky sister is Linnet, who is always playing tricks. The adventures are endless and there are more animals than you can count. This book is a creative mixture of the present with the past.
Let me unveil some secrets. The mysterious author, L. M. Boston, is really Lucy Maria Boston. She didn't start writing books until she was 62. This book is just the first book in the series. She really did own the oldest inhabited house in England. You can imagine how many centuries that can be.