This is one of the Clockwork Century books, the drug-zombie steampunk ones including Boneshaker and Dreadnought. It fills in and advances that world, showing us the effects of the creeping yellow gas on animals and even cryptids (cryptiiiiids!) and expanding our view of where things are and where they might go. There is arc plot here--while I think you could start with this story and be all right, it's much richer for having the preceding works to give it heft and ballast, to give the characters who are secondary characters in this book more backstory and more meaning for the reader than they have time to receive here. As series books go, I think this could work as a stand-alone--but it is a series book.
I think that's its major strength for me, because frankly I would have had zero patience for the new main character if he showed up and was my personal entry point to a new story and world. Rector is a whiny, resentful, entitled addict and small-time drug dealer. If you've already been following this world and the people in it, he is an entirely reasonable direction to go as the next facet to explore--but coming in cold, I don't think I could have stood him, not even to get more Houjin and Angeline. I am all for unsympathetic protagonists, but annoying protagonists are another thing entirely. Luckily the adventure structure of the book gets going fairly quickly, leaving Rector a great deal more time to develop as a human being and run around doing exciting stuff in the yellow nasty fog and a great deal less time grumbling and smoking sap than the opening of the book might imply.
Also did I mention cryptids. Because: cryptids.