In short, Laurie Halse Anderson's Wintergirls is a YA novel about an 18-year-old girl with anorexia who is relapsing for the third time after the death of her childhood best friend, who had bulimia. In short, not a pretty story.
I would think Readers' Advisory librarians would want to subtly make clear that this book could be a trigger for someone suffering from these disorders or very difficult to read for loved ones. To me, this is not a fault of the book, but just an extra layer one should be aware of when recommending it. But, if you're concerned about that, here's Jezebel and the Times on this book as a trigger.
From my perspective, telling a prospective reader it is a raw, emotional, terrifying, and realistic exploration of the actions and thoughts of a young girl suffering from anorexia and other psychological disorders would suffice.
The pacing is fast, characteristic of a YA novel. The language is carefully crafted to mimic the main character Lia's thought processes. Anderson uses strike throughs, repetition, paragraph breaks, and ellipses to evoke Lia's confusion and fear. (these tactics could also fall into the story line appeal element of RA--not sure here.)
Characterization is great. The book is a very intimate look at Lia; and her friend who died, Cassie, is evoked via Lia's detailed, episodic memories and hallucinations. The tone is dark, scary, horrific, touching at times. But Lia is witty, and, although it's typically in the form of barbs directed at her parents, she can be funny. The setting is winter in New Hampshire, and Lia is always cold--it is effective to think of her skin-and-bones body in this harsh climate.