The Suspect by Fiona Barton
Alex is saddened when her best friend backs out of a once-in-a-lifetime trip to Thailand. The 18-year-old doesn't want to travel alone. When another girl from her class offers to go in Mags' place, Alex goes against advice and accepts the offer.
When Alex and Rosie vanish, their parents fear something has happened. Police aren't as certain. They feel the teens are simply so caught up in traveling that they forgot to update their parents. Kate Waters, a British journalist, looks into the disappearance and realizes that there's far more to the case than anyone expects.
I wasn't taken by surprise. As you get to the know the characters through their emails and Kate's investigation, I could see what was coming. Despite that, I couldn't put The Suspect down. It was a truly gripping mystery.
This isn't the first book I've read by Fiona Barton. I've met Kate in a prior novel. The Child is equally addicting if you never read it. That said, while Kate appears in another book, The Suspect stands alone and is well worth reading.