This is a portal fantasy for middle grade readers, featuring two worlds, one in an alternate historical (1940s World War II) universe, with the possibility of many, many more.
The idea is that Tess' father uses a special device, a Star-Spinner, to deposit her as a baby on the steps of the Ackerbee Home for Lost and Foundlings, seconds before he's found by someone chasing him.
Now, the Home is, for a delightful change, not a horrible orphanage full of bullies and miscreants, but instead features a very kind and caring head, Miss Ackerbee, who has no qualms about Tess carrying around a tarantula to anchor her in the world. She's fully aware Tess disappears -- vanishes -- whenever the spider, Violet, isn't with her. Miss Ackerbee is also unusually astute for a head mistress in a middle grade book, and has figured out that Tess isn't from her world.
So when Norton F. Cleat comes to claim Tess and whisks her away to "Roedeer Lodge," Miss Ackerbee knows something's up, gives Tess the heads-up and spills the beans on all she knows about her, before letting Tess go.
It's with Cleat, however, that Tess finds an abandoned chapel and learns to use the Star-Spinner, which her father left on the orphanage steps with her. She encounters a boy her age, Thomas, who's lost his mother and father in another world, one ravaged by war -- a version of our WWII.
There she discovers Cleat's real intentions, and some of her family roots . But because there are many worlds, like the pages of a book stacked on high, she may never know exactly which one she's from.
The author bases the attack in the book on a real event, a 1941 Luftwaffe attack on Dublin, although she fictionalizes it. I wouldn't consider the book historical fiction, however, as it's clearly fantasy, and there's a set up for additional books to continue the story and each of those "realities" would have to be different than ours.
Enjoyed the read!