Pretty obviously, the title of the book—Fallen—refers to fallen angels. Shocking, we know.
But not so fast—we're not done yet.
Not only does the idea of falling relate to the literal nosedive the angels took from Heaven, but it also relates to the metaphorical feeling of falling in love, which Luce clearly does willfully and almost gleefully. This type of falling might not leave a bruise, but it can leave its own invisible scars, and unfortunately for Luce, she doesn't have wings to stick the landing.