Gossamer by Lois Lowry

Littlest One is in training as a dream-giver, a delicate creature whose job it is to turn memories into dreams that give comfort and hope to humans. Thin Eldest is her teacher, doing his best to guide Littlest as she learns to use her considerable gift for bestowing dreams on the inhabitants of their assigned house--an abused 8 year old boy and his elderly foster mother. Comforting the angry boy is a task made urgent when a Sinisteeds begins to inflict nightmares on him. When the Sinisteeds gather to attack the child's sleep in a horde, Littlest must use her ingenuity to save him. Lowry skillfully weaves together all the strands of the story--the sweet, caring work of the dream-givers, the violent thoughts and words of the boy, the determination of his mother to make a home for him, the calm patience of the foster mother--into an allegorical story that can be enjoyed on more than one level.