Cicada Summer by Andrea Beaty
Eleven-year-old Lily has a passion for Nancy Drew stories and a secret she is keeping from those she loves. When summer brings lying, stealing, sneaky Tinny Bridges to town, Lily must be on her guard with this perceptive newcomer, or risk having her secret revealed. But Tinny won't leave Lily alone. She takes candy from the general store and blames Lily. She tries to steal Lily's friends and even her father's affection. Then Tinny goes missing, and only watchful, mystery-loving Lily has any idea what happened to her. But for Lily, finding Tinny means confronting her hidden past.