The Thirteenth is Magic by Joan Howard (story of siblings who live in a New York City apartment building where - mysteriously - there is no 13th floor) (the fact that used copies are over $100 might indicate there is indeed demand!)
Emmy Keeps a Promise by Madye Lee Chastain (19th century story of sisters Emmy and Arabel who come to New York City to make their fortune - or, at least, Arabel comes to teach in a private school for young ladies and Emmy comes to keep an eye on Arabel)
The Lark in the Morn by Elfrida Vipont (first of two books about Kit Haverard, the "untalented" child of a musical Quaker family who finally discovers where she fits in)
I would have included the Mushroom Planet books except that the first one is still in print. My nephew liked the first one but it may be hard to get him all the sequels. I remember selling a tired ex-library copy on eBay a few years ago for $50.
If I could include adult books, I would start with Elswyth Thane's Williamsburg novels, of course. They desperately need to be introduced to a new generation. They might be my all time favorites (a strong statement, indeed!).
Any other suggestions?