I've composed four drafts of this review and three were all over 2,000 words. Since that's insane, I deleted them. That's a terrible lede, but stay with me. Let's just say that my adoration for Prosperity knows no bounds, but the difficulty I had with reviewing it wasn't a result of fangirl squees (though which would be totally appropriate here as I loved it that much). If you're looking for an unqualified recommendation though, this is it: the world-building is spectacular, the characters vividly drawn, the romantic arc singular and the emotional impact staggering. You should buy and read it immediately.
The main character and narrator, Picadilly (Dil for short) is a Dickensian sort of pseudohero, an imperfect, undereducated, brilliant, overconfident grifter of a fellow. Other characters include a handsome, talented, oblivious former man of the cloth named Ruben, an elegant professional thug referred to as Milord, a genderqueer skyship captain named Byron Kae, and an aethereal navigatrix named Miss Grey. The setting is pure "London Above"* steampunk Victoriana, filled with floating cities, skyships and krakens. This cast of characters interacts in various ways throughout their adventures to collect phlogiston (a process I imagined as similar to the lightning-gathering scenes in Neil Gaiman's Stardust), avoid fearsome krakens and sky pirates, and eventually attempt to save Prosperity itself.
Of course, that doesn't begin to till the emotional or thematic ground the novel covers. Generally speaking, the novel explores the problem of what happens when people whose values don't align fall in love. The crises in the book precipitate shifts in those values, but it doesn't explicitly reward the shifts in the way of traditional romance novel didacticism. There's a measure of redemption for all the characters who needed it, but for the two I liked best, it's like they were both given new shoes that are a few sizes too big still. There is more growth yet to come, I think.
Some day I may find voice and space to talk more about all of that, maybe at the end of the series. But for now, I'll just say that for romance readers, I'd call this ending an HFN and I'd expect some people to disagree with me. At the very least, those who insist on one happy ending per couple per book may be frustrated by Prosperity. But if you can look past that or you're willing to wait for the entire series first, I suspect all will be well.
It's not an easy book, but it's one of the best I've ever read, in or out of the romance genre. I'm very much looking forward to the rest of the series.
*A reference to Neil Gaiman's "London Below" in the novel/miniseries/radio play Neverwhere.
EDITED TO ADD: I received a copy of Prosperity from the publisher for review purposes.