No One You Know by Michelle Richmond.
When Ellie Enderlin's sister Lila, a student at Stanford, was murdered, Ellie poured out her heart to a friend who turned the story into a true crime bestseller. She never forgives the man for using her, and shuts him out of her life.
Twenty years later, Ellie encounters the man who was accused (but not convicted) of her sister's murder, and he gives her Lila's journal. As Ellie reads it, she becomes more and more intrigued, and decides she's going to find out what really happened.
This book is written from Ellie's point of view, as she talks to people, and tries to piece together what happened all those years ago.
Ellie is a strong, likeable protagonist, who is able to look at her younger self quite objectively, and the reader is drawn into her quest.
Richmond has crafted a riveting tale, populated by believable characters, and I plan to find some of her previous novels. This book is one of the best I've read this year.