Archer's Goon
Diana Wynne Jones1984

Thirteen-year-old Howard Sykes lives in an English town with his parents, Quentin, an author and professor, and Catriona, a music teacher; his sister Anthea, always called "Awful" because of her constant screaming; and Fifi, the family's au pair. Their life is interrupted one afternoon when an unnamed huge person, "somebody's Goon" as Fifi describes him, comes into their home and announces that he has come to collect the two thousand words that Quentin owes somebody called Archer.