Entertainment Weekly shared an interview they did with Cassandra Clare and an excerpt from City of Heavenly Fire.
Clare spoke about the cover, some of the details on it and the inside of the cover jacket.
“I really love this cover. (…) We talked back in the beginning about deciding who was going to go on what cover, and that this cover was going to end up with Clary and her brother. This is the big face-off. This is the book where she goes up against her brother Sebastian, and he is the big villain. It’s kind of like a Batman/Joker cover. … And in a way, there’s a play of light and dark going on on the cover, and that sets up a big battle between good and evil with Clary standing for good and Sebastian standing for evil. There’s also little clues for the readers who have been reading the series a long time in the weapons that they’re holding, in the necklace that Clary is wearing, and especially in the landscape that they’re standing on. It gives a hint about what’s coming and what’s going to happen in the book.”
“This cover is exciting and really beautiful, but a really cool thing about it that hasn’t been revealed yet is that it actually wraps around to a whole back of the jacket. If you take the jacket off, the whole image wraps around to an interior image that shows all of the rest of the other characters. They’ve never gotten their own covers, so this is a chance to see them for the last time. I think it’s something that people are really going to love. It makes the jacket a piece of art.”