Fires of the Ancients (YA SciFi):

A floating city which was designed to survive the apocalypse is now crumbling due to its age. The City doesn't have much time left, and it will take two teenagers, a magic necklace, and an unusual book to save it.

Poh (MG SciFi)

A young girl finds an unusual stone near the river, but quickly finds out that it could signify the end of her family's refuge from an alien assault.