Wintergirls, by Laurie Halse Anderson.
Cassie and Lia were best friends since fifth grade, reading fantasies and ensnared by dangerous habits. By the time Cassie died alone in a hotel room, though, Lia hadn’t seen her for six months. Cassie had moved on to new friends, and maybe a little too far. That last cold night, she tried to call Lia’s cellphone 33 times. When the calls didn’t work, the hauntings started. As Lia’s anorexia intensified, so did Cassie’s visits, urging her to stay strong, looking forward to joining her once more. Is this real, or is Lia dreaming her way down Cassie’s lethal spiral?