The question is whether a book describing anorexia will make girls anorexic.

Oh, good lord. By that logic, I should be a Victorian serial killer, a brain surgeon, a gay male, a wizard, autistic, bipolar, even more OCD than I already am (I mean, why just wash my hands excessively when I can actually scrub the skin off?), a prince of Amber and quite a few other things.

In reality, a book like that could cause someone to recognize themself and their problem and get help just as easily as trigger someone.