The Ruins by Scott Smith

Trapped in the Mexican jungle, a group of friends stumble upon a creeping horror unlike anything they could ever imagine. Two young couples are on a lazy Mexican vacation–sun-drenched days, drunken nights, making friends with fellow tourists. When the brother of one of those friends disappears, they decide to venture into the jungle to look for him. What started out as a fun day-trip slowly spirals into a nightmare when they find an ancient ruins site . . . and the terrifying presence that lurks there.