Vanishing Girls by Lauren Oliver
Sisters Dara and Nick used to be best friends, but after "the accident" they are complete strangers, living in the same house. Nick is spending the summer working at the local amusement park, and trying to get her life back to the way it was before the accident. When a local 9 year old goes missing, Nick feels compelled to find her, and discovers truths about her own family in the process.
Dark and atmospheric, readers won't want to put this one down (plan accordingly, and expect to read this in one sitting!) Oliver is a master at jumping around in the timeline of the novel, revealing little snippets of information as needed.
Readers who enjoy Vanishing Girls should also check out Lauren Oliver's other contemporary YA novels Before I Fall and Panic, along with her adult debut Rooms. And for those who are looking for novels with unreliable narrators, I'd also recommend E. Lockhart's We Were Liars, and Liar by Justine Larbalestier.