Skybreaker and Starclimber by Kenneth Oppel

Serious adventure- you can visualize every scene like it was a movie (hopefully it will be soon!). The alternate universe they live in is fascinating and the characters are fun. New characters in Skybreaker, Nadira and Hal add to the story. Starclimber was my favorite- the science was fascinating (Yes, actually, it was!) and the romance between Kate and Matt felt realistic. I have even gotten my mother reading them!