Heat Stroke is the 2nd book in the Weather Wardens series by Rachel Caine. If you haven't read this one or the first book, don't read any further!
This one started off a little slow for me, but the action of the 2nd half made up for it! Joanne is now a Djinn, and David is trying to teach her the ropes. What she doesn't know is what he had to go through to create her. He is losing strength to keep her strong, and there is so much more that she isn't aware of...
Again, at times there was so much going on that I wondered if I was really comprehending it all. A little of it might have been over my head, but I did enjoy the story and I was surprised at how it ended...although I did wonder how long she'd be able to survive as a Djinn (I think there are 7 or 8 books in this series). It will be interesting to see how she handles being a human again, with David still as a Djinn. The ending definitely leaves you hanging!