The Secret Museum, by Sheila Greenwald (1974), which I just read in a matter of minutes, was a mere 127 pages, though written for upper elementary/middle grade kids, which is about 100 pages less than books for that age being written today...and yet I can't really see that it needed more to it.
It's the story of a girl named Jennifer, whose parents quit their city jobs and moved to an old house in the country where they were going to make a living selling pottery and textiles. Her parents, however, are much better at their art than they are at marketing, and financial disaster looms...
Jennifer, out on her own picking berries in the shut-up old estate nearby, hears crying...and follows the sound to an old playhouse (of the lavish kind that goes with old estates). Inside, she finds twenty or so beautiful dolls, abandoned and filthy. And one of them was crying, and they can talk, and Jennifer comes back and washes them while they are talking to her and I think it is REALLY CREEPY to wash a sentient doll, but she does it anyway. (I think the dolls' clothing would have been a lot more mouse-eaten than it was, but that I can forgive).
And then Jennifer meets another girl, Lizzie, who is first enemy and then friend, who has the idea of turning the playhouse into a doll museum, and they paint and clean (I liked this part lots; it was outside work, so the dolls didn't have the chance to talk), and they advertise, thanks to Lizzie's gumption in this area, and it is a success. Except, of course, the house and dolls don't belong to them...The old lady who once played with them, and then abandoned the whole estate, is alive and well and deeply annoyed when she finds out what they are doing.
But it all works out, and Jennifer's parents are inspired by her example to sell their own products more aggressively, and we learn the lesson that you can make your own luck and that multiple signs are better than one obscure one, no matter how beautiful your art. (I am always open to Learning Lessons from Books, and shall advertise the next library book sale more aggressively).
I would have loved this book except that, as noted above, the fact the dolls talked gave me the creeps, and it made me cross because their talking wasn't necessary for the book to work and was, in fact, totally gratuitous. However, that could just be me having Issues, and in general I think it is an excellent one to give a seven or eight year old girl who loves dolls.
The Secret Museum is available quite cheaply, and is still in the library system of Rhode Island (I will be returning it on Monday, d.v.).