The Bromeliad Trilogy by Terry Pratchett

From chapters.ca:
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.