Loved this. 
 Harri has recently emigrated to London from Ghana with one of his two sisters and his mom; the father and sibling are coming later. He's eleven and discovering the ins and outs of urban gangs, middle school crushes, sport. It's a pretty rich ground in which to plant a good mystery. 
 Harri and friend Dean are not what you'd call typical detectives, of course, being guided by popular television, but the unsolved stabbing of a neighborhood boy nags at their sense of justice. As does the hope of reward. 
 But the larger mystery is this transplanted boy trying to figure out life. It has emotional heft and a great drive. 
 This is the third book to make me cry in 2011. Getting soft.