This is the conclusion of the Onyx Court novels--at least for now, and I think it makes a pretty thorough conclusion. Even if swan_tower decides to do more novels in this universe, it seems like these four make a series and another thing would have to make another series. Each of them stands to a certain extent alone, but for those of you who complain that too many fantasy series these days go on and on and do not end: here you go. This ends. Each novel has a stand-alone ending, and the series also does. And very satisfyingly at that.
Each Onyx Court book takes a period of English--specifically London--history. The two middle ones are "mine," the Stuart/Interregnum book and the Hanoverian one, but this latest one is very good anyway, even if it is less thoroughly "for me" (it's Victorian). This one reverses the pattern of previous books by focusing on a human woman and a Faerie man instead of having the opposite race/gender combinations.
The emphasis on Faerie science is not nearly as strong as in A Star Shall Fall, but there's still a strong interest in characters who need and want to figure out how things work, who are curious about their world and who are sometimes wrong. While the consequences of the first three books definitely reverberate into this one, I think I've changed my mind about how much they stand alone. I think that you could read this one without Midnight Never Come and the other two. You'll get more of the characters if you've read them, certainly; the impact of the thing will be greater. But the Jacquard looms and Babbage engines of Faerie will still make sense to you and make a whole book if you haven't had the Great Fire first.
My main quibbles with this book are on the level of quibbles rather than actual complaints. First, I would have liked more Ada Lovelace, as in fact who would not, but books cannot be infinitely expanded to fit historical figures to my tastes. And second I felt that the epilogue set the series on a rather different footing than it had been on. It was not a bad new footing per se, but I liked the previous assumptions/relation to our world a bit better. (But since it's in the last five pages, even if I had found it to be outright bad--which, again, I did not--it still had relatively little impact on the series as a whole.)
This weekend at 4th St. I once again ran into people who were claiming that they had been burned by fantasy series that go on and on and that they would not buy a series until it was complete. So be advised: this one is complete, it has an actual ending, and it remains excellent throughout. Highly recommended.