"Yesterday's books for today's reader," it says right there on their webpage; also there, Wilmar H. Shiras's Children Of The Atom. The book -- more a collection of closely related novellas -- opens with In Hiding, a deeply moving story about a child who is more than he seems and probably the most anthologized work by Ms. Shiras. Interested? You can order the book via the Amazon link at Tam's -- it costs you just the same but helps her out.
The book is uncommon at used-book stores despite at least four editions (Gnome Press, a delightfully illustrated version from Pennyfarthing Press, Nelson Doubleday and now Red Jacket Press), possibly because, like Terry Pratchett's books, those who have a copy, keep it. If you happen to own the Pennyfarthing Press edition, you'll find a few drawings include what could well be a young Roberta X: a little girl in big, round eyeglasses, hair pulled back, sitting tailor-fashion with a book in her lap, looking out at the world with a skeptical expression.
I stumbled onto the publisher by way of Frederik Pohl's blog, q.v..
1. Late -- gone, in fact -- of San Francisco and not to be confused with comic-book publisher Penny-Farthing Press
2. Though the text describes her as "plump," the artist didn't see her that way. The character is every bit as notional as I was at that age, though altogether more talented and accomplished.