Beside Myself by Ann Morgan Book Summary:
Six-year-old Helen and Ellie are identical twins, but Helen is smarter, more popular, and their mother's favorite. Ellie, on the other hand, requires special instruction at school, is friendless, and is punished at every turn. Until they decide to swap places--just for fun, and just for one day--and Ellie refuses to switch back. Everything of Helen's, from her toys to her friends to her identity, now belongs to her sister. With those around her oblivious to her plight, the girl who used to be Helen loses her sense of self and withdraws into a spiral of behavioral problems, delinquency, and mental illness. In time, she's not even sure of her memory of the switch. Twenty-five years later, she receives a call that threatens to pull her back into her sister's dangerous orbit. Will she take this chance to face her past?