'Undone' is the first in the series of Rachel Caine's 'Outcast Season', a spin-off of the Weather Wardens. I strongly recommend reading them in publication order before reading this one, so that would entail reading the first seven WW. I think the book would make more sense, you'd have a better idea of the world and it wouldn't give any spoilers away when you come to read the WW (I think the twists in the series is one of its best traits). 
 Okay, so Cassiel is a Djinn who has been cast out by the head of the Old Djinn and who needs the Wardens to survive. She is completely different to Joanne (who does make an appearance which was fun), arrogant and quite alien I suppose - I liked seeing her character progression through the book. Not sure about Luis yet, I don't feel I know him enough to truly love him at the moment but I'm hoping that will come in the next book. 
 The gangs were a weird addition. Then we got back on track with the ginormous-end-of-world-threat. 
 Do I like it as much as the Weather Wardens? No, but I'm not too worried about that since it took me a few books to really get into them anyway. With Rachel Caine, I think it takes a while to get used to her writing. I'm not talking about the writing style, I'm talking about the content. She is not afraid to go all out on ideas and I like that (at first I was a little unsure). It's almost like an epic fantasy... set in the real world... but different. I also like that she's not afraid to kill off some characters. 
 I will be reading the next one hopefully soon after I've read Cape Storm