Skin and Bones by Sherry Shahan

Sixteen-year-old Jack, nicknamed "Bones," won't eat. His roommate in the eating disorder ward has the opposite problem and proudly goes by the nickname "Lard." They become friends despite Bones's initial reluctance. When Bones meets Alice, a dangerously thin dancer who loves to break the rules, he lets his guard down even more. Soon Bones is so obsessed with Alice that he's willing to risk everything–even his recovery.