Book 5 in the Gone series by Michael Grant.
Night is falling in the FAYZ. It has been nearly a year since everyone over the age of fifteen disappeared. And even though everything changed, the sun continued to shine on those left in Perdido Beach.
But now the gaiaphage has blotted out the sun and plunged the FAYZ into perpetual gloom. Divided and dispirited, the survivors face their greatest enemy yet - the darkness of their own minds.