I think the other reviewers covered most of what I liked about this book. I am really enjoying the amount of material available for Varisia, since I'm planning on doing Rise of the Runelords, Jade Regent, and Shattered Star as kind of a loose 'Varisian' trilogy eventually. Things like this allow me to add in those little details that make a game come alive. I'm already worried about getting lost in Magnimar when I do Rise of the Runelords - there's just a lot of potential for adventure hooks here.
Let me also say that the artwork of the notable NPCs is just amazing, especially the portraits. The cartography looks great as well. If the quality of the campaign settings continues to impress, I'm going to start picking these up every time a new one comes along. If Paizo keeps doing these so well, we could potentially have one of the greatest RPG settings of all time out there with the Inner Sea, Avistan and Golarion. Nice work.
I've read quite a few Pathfinder Tales books, and while I liked some better than others, all of them leave me with what is probably the intended purpose of these tie-in novels - to get your imagination fired up about things you would like to include in your games, and some character types you might not have thought of.
To be honest, the Alchemist class didn't immediately interest me, but this book changed my mind a bit. Alaeron, the main character, was interesting and well-written.
The plot sort of revolves around the journey that Alaeron and his unlikely companions take to an ancient ruin on the edge of the Mwangi Expanse, which used to be a floating city of an ancient race. The character development is good, and the cat and mouse chase subplot was interesting as well. In fact, I enjoyed the book quite a bit until I got to the end, where I really felt like the author wanted to elaborate on the destination a bit more, but might have run up against a deadline. There were some nice little 'twists' at the end that would have been nice if they had been elaborated on a bit. Still, it was fantastic stuff, and anybody who wants to know about the Shory, Numeria (it's a big factor in a major sub-plot, and Numerian relics from the Silver Mount play a big role in the story,) Andoran, and of course derhii (flying gorillas! awesome) should pick this up. A good read.
I'd also like to mention that the author including a gay character in the novel that doesn't play to stereotypes is great, and I'd like to see more of that in both Paizo's fiction and popular literature/movies/television in general.