I personally have not read the whole book but from what I can tell the Title winter girls refers to the two main characters Lia and Cassie who both have eating disorders and one end up dying from the disease competing against each other they let this overcome one another I think Winter girls relate to the whole idea that these two girls have this secret and no one knows how alone and cold the pain this disease is bring to them which eventually leads to death. There is a quote the author uses that kind of explains the title more it is you are not dead but you are not alive you’re a winter girl…..it’s like this is what the author thinks young women who also battle with the same thing are symbolic to. Blinded by the competitive dysfunctional desire it leads to real death not only is this hard but its sick it is truly something young women should be aware of!