Martha Wells seems to be struggling recently. While her early books were great, later offerings have been much weaker - not bad, but not up to the previous standard. Sadly, Emilie and Hollow World continues the trend.
Emilie is clearly written as a Young Adult novel, with a naive but plucky young heroine encountering the cruel world for the first time, and with many opportunities for her innate goodness to shine through. That's all well and good. What's disappointing is that Wells seems to be so consciously adhering to a formula. Every moment feels carefully scripted and targeted. The result is a book that's pleasant, but not especially interesting.
Despite the care, Wells seems to miss her target. We don't learn Emilie's age for a couple of chapters, and when we do, it's surprising. I had the feeling of a 10-12 year old, not an adolescent. Even when we learn Emilie is nearer being a young woman than a child, she doesn't feel like one. The introduction of a love interest is thus a bit offputting. Perhaps, in a world where most fictional teens face harsh dystopias at every turn, there's room for a return to starry-eyed childhood innocence, but it didn't work for me here.
Wells' world is a similar return to days of yore, with Emilie and crew visiting the hollow center of the Earth on aetheric currents. I assume this is in conscious homage to Verne and Burroughs, and took no issue with the sheer scientific improbability of the setting, though there's little attempt to explain it, and some of the background details are improbable even within the context.
Wells does better with characterization. She winks at historical stereotypes by quietly inverting some gender roles, but sets the story in an Elizabethan-era steampunk setting in which woman are meant to play a submissive role. This gives Emilie something to rebel against and overcome, but it feels a bit tired. Emilie herself is an active, intelligent protagonist. She worries and wonders, but takes action - not always in very credible ways. She faces and overcomes fairly predictable challenges.
Altogether, this is a quick, modest adventure for kids who haven't read too widely (and thus encountered most of the ideas already). There's nothing wrong with it, but also not anything particularly compelling. I don't feel any need to go on to the next book.