During a recent trip to SF, my friends and I stumbled upon the Children's Book Gallery on Geary St. It's located in a large building along with several fine art galleries, but no one in the building seemed to even know it was there (I saw its name on the directory in the lobby and had to investigate).
It was adorable, and the owner seemed happy to have people in there who appreciated it. Everything is for sale, so it's less of a gallery, and more of a very specific antique shop.
She has books from 1850 to 1950 mostly, and the design and pictures are priceless.
Here are two of my favorites:
I got the impression that she did most of her business online, but it's sweet that she has set up this little gallery space to display the books. They are beautiful and fragile and deserve respect, so it's nice to see them in a space where they get that.