The listing, The Ruins by Scott Smith has ended.
2007 paperback edition looks excellent.
The Ruins follows two American couples, just out of college, enjoying a pleasant, lazy beach holiday together in Mexico, as on an impulse, they go off with newfound friends in search of one of their group; the young German, who, in pursuit of a girl, has headed for the remote Mayan ruins, site of a fabled archeological dig. As the searchers move into the wild interior, they begin to suspect that there is an insidious, horrific "other" among them .