The copy I checked out has a "mystery" label on it, and I think you could just as easily mark it "adventure." I'd even argue that the mention of witches in the title is designed to make you think it's a fantasy. (Although that 2009 cover really gives it away, huh? Way to put the final dramatic scene right on the front of the book.)
Librarian issues aside, I really enjoyed this book. The beginning captured me, because it described two kids from a tough, city neighborhood being taken to a group home because the old woman who had taken care of them for 10 years was hit by a car. Not only are Tony and Tia orphans, but they have no idea who their parents were or how they ended up with Granny Malone. With "olive" skin, blonde hair, and blue eyes, they don't look like any nationality, and they have some strange powers, like telekinesis and the ability to see and hear across long distances, that they have learned to hide.
This is the kind of hard-luck story kids love, and I thought that the urban setting, although it wasn't described with any love, would be familiar to my students. Sure, the language is a little dated, but change "grifters and confidence men" for "hustlers and gangsters," and you're talking about a place my students would recognize. Point kids to the scene in which the kingpin of the group home attacks Tony with a shiv--and Tony takes him out with a pillow and a few swift kicks--and readers will be sold.
But there's something else Tony and Tia can't fight: a network of very official adults who want to give custody of the children to a sinister man who insists he is their uncle. The children know in their bones he's lying, so they make a midnight escape from the group home and bang on the door of the local priest. Hey, remember when priests were associated with kindness to the poor rather than child sex abuse scandals? Anyway, Father O'Day is a very Madeline L'Engle kind of priest, and his spirituality gives him the ability to believe in the children's strange story and even stranger abilities.
One thing that struck me about the book is its strong rural bias. Not only is the city portrayed as a savage place, but there's more than one comment about how, when the children see forests and fields and mountains for the first time, they have a feeling of joy and homecoming. On the other hand, rural people are not portrayed very kindly. They are almost exclusively pitchfork and gun-wielding witch-hunters who are just as greedy and suspicious as city people. I suppose most of the adults in the book are pretty bad dudes.
So, as I mentioned in my review of The White Mountains, I'm rereading children's sci-fi classics to see which would be good additions to the sci-fi section in my elementary school library. And this one's a keeper! The book reads fast, and the dated bits shouldn't trip students up. Personally, I laughed when Tia mentioned she wasn't allowed to wear skirts in the group home, and I skimmed when Father O'Day started talking about Communists and Hungary. Actually, there's just one sentence about Communism. There's a much longer attack on capitalism, which made me wonder what kind of economy the author supported, but like I said, the author doesn't spend much time on these ideas.
However, one of my concerns about The White Mountains was that the technology in it feels so dated it makes the story a little confusing. Escape to Witch Mountain doesn't suffer from this, because we barely even see the tech. There's just references to ships and stars and the appearance of one flying saucer. And the aliens in the book have truly impressive powers, unlike the tripods in The White Mountains, who are menacing, but kind of clunky. So I'm definitely buying the book for my collection ... I just have to figure out whether or not to put it on the sci-fi shelf.