(I got an ARC copy from NetGalley. At the time, the book wasn't published yet, but it took me some time to get to it. Some things may have changed between the version I have, and the actual, final version.)
One thing I have to get out of the way: I didn't read Emilie and the Hollow World, the first installment in this series. Although each story seems to be self-contained, it was obvious that some elements from book #1 permeated book #2, partly through brief recapping here and there; and so I may or may have not missed a few things.
This story follows a classical adventure format, its world and themes reminiscent of classical works as well (the—lovely—cover, among other things, made me think of the old Jules Vernes books I had when I was a child). Airships, explorers, scientists with a dash of magic to their craft, and young people wanting part of the big adventure: these can and will certainly appeal to a younger audience. Bonus points, also, for the aether-ship creature, who truly was alien (not humanoid, as in too many novels), and for including the communication/language barrier. I tend to be highly skeptical of "aliens" with whom communicating is awfully easy in every aspect.
On the other hand, I found the story easily predictable
and I think even younger readers would have the same feeling. The chapters leading to getting into the aether currents were too slow to my liking; I guess I wanted the Big Adventure to start faster—that's why I pick adventure stories. (As I mentioned, I didn't read book #1, but I suppose someone who has would want things to start faster?)
But those are smaller qualms: after all, sometimes we just want predictable, and in such genres, it's not that much of a problem per se. The real issue for me were the characters, whom I just didn't connect with: they felt brushed over, shown on the surface only, when clearly something deeper was going on for some of them, and I would've liked to see more in that regard. Emilie struck me as too mean and petulant regarding her brother; granted, they seemed to share history, but they're still young (Efrain must be, what, 10?), and it was weird, because they somehow felt like they shared twenty years of resentment. As a result, Emilie wasn't particularly likeable—at least, not like the adventurous, smart and resourceful girl I imagined her to be when I started reading. Same with the Professor and Miss Marlende: the tension between them was never really explained (though I could feel very early who they were), and it would've been interesting to know how exactly things became that way.
In itself, it was a fairly nice story, but one I'll probably forget fast