Vickery and Castine is a series that I liken to in my mind to Another Kingdom by Andrew Klavan. I'm not 100% sure why exactly this is, but I do at least have a few guesses. Firstly, a significant chunk of the books (effectively all of this one and approximately half of "Another Kingdom") take place in the modern world, in a familiar version of America. To contrast these modern inclinations, both have their own fair share of the fantastical. For Another Kingdom, this is quite literally in the form of an actual fantasy world connected to the real world a la Narnia; for Vickery and Castine, the fantastical comes in the form of alternate, ghost-dimension kind of stuff...amongst a lot of other frankly strange stuff that's really hard to explain. It's some cool stuff don't get me wrong, but it *is* definitely strange, and goes so far as to be off-putting at times - perhaps one of the things I have the weirdest relationship with in the books.
That said, all three of the Vickery and Castine books delve into different realms of strange: there's supernatural stuff, there's other-worldly stuff closely associated to certain historical mythologies, there's funky time-based jargon, and there's alien stuff, depending on which book you're talking about. "Stolen Skies" happens to be the alien stuff, and it is significantly more awesome than anything from the 2 previous entries in my opinion. As a result of this content, "Stolen Skies" is (oddly enough) the most grounded or "normal" of these 3 books, and this is a lot of why I think it is the strongest part of the trilogy for me. That's not necessarily to say it's an absolutely amazing book mind you, but if there was ever anything in the first two that you considered to be good or great, "Stolen Skies" takes the good stuff, leaves the not-so-good behind, and plays consistently quick and fun from start to end.
But let me back up just a bit. I made the Another Kingdom comparison in the first paragraph, and kind of left that idea hanging. Why, beyond the real-world-mixed-with-the-fantastical similarities, do I compare these 2 trilogies. Well...again, I'm not 100% sure, but I think the rest of it comes down to the fact that both are trilogies, both have notable cases of middle-book syndrome, and both have 3rd books that wowed me at times with certain ideas while the first 2 were either just pretty good or mediocre (respectively for books 1 and 2 and also in accordance to both series). Thus I have connected the 2 in my mind.
For Another Kingdom: "Another Kingdom" was super solid and a book I really liked. "The Nightmare Feast" was largely forgettable beyond a couple scenes. "The Emperor's Sword" is still a personal favorite.
For Vickery and Castine: "Alternate Routes" was a solid introduction that got me hooked...but then kinda lost me in the final act. "Forced Perspectives" had a lot of really cool ideas that definitely were not executed well on for my personal liking. "Stolen Skies" isn't quite a personal favorite, but was worth reading the first 2 books to experience.
In either case, I had a book I thought was pretty good, one I didn't like too much, and another I enjoyed quite a bit.
Where the two are separated in my mind however, is actually the same place where the original similarity comes from: the fantastical intermixed in the real world.
Vickery and Castine is very strange in many regards. I've briefly listed a handful of the main things, without getting into specifics. The first book, and a bit of the second, messes the most with mythological kinds of things. The second book, and a bit of the third, messes the most with time-based stuff. The third book is the alien stuff, as I mentioned. Lots of cool ideas sprinkled throughout, all connected by the main supernatural concept of the series: the ghost-dimension type stuff.
Ultimately, these books are made and broken by these things. The first book mostly lost me with the final act, where the combination of ghost-stuff and mythological-stuff just kind of ballooned in ways I don't think I was ready for. The second book mostly had too much of the rules and repetition associated with the ghost-stuff (not to mention being too long in the first place) that irked me slowly more and more over time. The third book went all in on alien stuff, and it was actually super great, leaving behind the more repetitive and annoying nonsense of the first two.
So that's the basics. In addition to most sound executing on the supernatural elements included, "Stolen Skies" has the strongest display of side-characters, and uses the supernatural the most naturally in the course of the story. Perhaps this is because the things Powers has done with aliens in this entry are, well, the most involved in the modern cultural zeitgeist, so things just worked better; I definitely think it is. Where most of the ideas of the first two required a bit too much explaining to get (or were too repetitively used, such as basic math being used to ground ghosts in reality in order to talk to them), "Stolen Skies" was made by it's creativity where the first two were hampered just as much as they weren't.
I also found the plotting to be the strongest here. Though the first half of "Alternate Routes" was quite solid, I did fall off because things took a turn (not a plot twist, just the way the story was running); "Forced Perspectives" was definitely a bit too long; "Stolen Skies" was paced the best, and moved along through the best material at the best speed.
This may not be a masterpiece of a trilogy, but it was decent fun. I don't love the first two books, but I did quite enjoy this one. I won't say this is where you should start with Powers like I did, because I don't think that's true, but if you love Powers, maybe you'll love it anyway. After all, these books have basically the highest average scores of any of his books here on Goodreads.