The Great Gilly Hopkins, by Katherine Paterson, 1987

It takes a particular genius to terrorize an entire county's foster care and social services systems, but 11-year old Galadriel, aka Gilly, Hopkins, is the girl for the job. When she arrives at Mamie Trotter's home, her bag of tricks to manipulate, antagonize, and escape is fully loaded. But all she really wants is for her mother, Courtney Hopkins, to come for her. Gilly is a wonderfully complex and convincing rascal heroine, whose ingenuity is only matched by her longing for home. Strongly recommended, even if the ending made me throw a Gilly-style fit.