The Bromeliad Trilogy: Wings

In a world whose seasons are defined by Christmas sales and Spring Fashions, hundreds of tiny nomes live in the corners and crannies of a human-run department store. They have made their homes beneath the floorboards for generations and no longer remember—or even believe in—life beyond the Store walls.
Until the day a small band of nomes arrives at the Store from the Outside. Led by a young nome named Masklin, the Outsiders carry a mysterious black box (called the Thing), and they deliver devastating news: In twenty-one days, the Store will be destroyed.
Now all the nomes must learn to work together, and they must learn to think—and to think BIG.
Part satire, part parable, and part adventure story par excellence, master storyteller Terry Pratchett's conclusion of the engaging Bromeliad trilogy traces the nomes' flight and search for safety, a search that leads them to discover their own astonishing origins and takes them beyond their wildest dreams.