Monday's Not Coming by Tiffany Jackson
Monday Charles is missing, and only Claudia seems to notice. Claudia and Monday have always been inseparable—more sisters than friends. So when Monday doesn’t turn up for the first day of school, Claudia’s worried. As Claudia digs deeper into her friend’s disappearance, she discovers that no one seems to remember the last time they saw Monday. How can a teenage girl just vanish without anyone noticing that she’s gone?
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