It's called "The Ruins" by Scott Smith. It's a paperback and over 500 pages but once I started it, I couldn't stop and read it in two days. I was still creeped out a couple of days later. It's not a slasher type book but a subtle creepy book. I read where he had another book on the market and I'm trying to find it.
Anyway, if you're looking for a most excellent read, this is the one!