The Key that Swallowed Joey Pigza is the last chapter in the Joey Pigza series by the incredible Jack Gantos.  I have written a lot about him here.
This book was certainly dark, it brought me to tears several times.  It is so rich and incredible and Gantos makes Joey come alive in a way that just opens your heart to him.  There were times I was so angry with his parents.  Okay, well I was angry at his parents all of the time.  But not Joey.  Joey is just such a good, good, good person.  He makes me want to be a better person.  Gantos reads the audio book and I love hearing his voice.  And he has a special message at the end of the book, talking about how he was inspired to write the Joey books because of a boy he met on a school visit.  He encourages all of us to give the kid who is off the wall or spinning out of control a chance.  Wow.  This whole book is just incredible.