E.L. Konigsburg really comes through with this one. Beware, this isn't a book for younger readers. Branwell- a slightly odd, very precocious child- stops talking the day he calls 911. His sister Nikki is in a coma, and the au pair (the icky Vivian) explains that Bran shook and then dropped her. Bran is put in a juvenile detention center, and it's up to his best friend, Connor, to develop a code and find out what really happened that day.
This book is really, really good.