I’m looking for a book that was written probably in the 1960’s, possibly early 1970’s. I think the title contained the words “secret friends”. It is about a girl and her best friend who stumble across a mystery involving 2, or maybe 3, mechanical dolls. Her father owns an antique store. He has one of the dolls, a girl sitting at a desk who dips a pen in an inkwell and writes. The other is a girl playing the spinet. I remember the clue to the mystery was found inside the doll.