The Hidden follows Englishman Ben Mercer as he flees a broken relationship and graduate school in archaeology/history, and heads to Greece. A series of happenstance encounters and decisions lead him to being on a dig in Sparta, chasing the ghosts of the Spartans of Thermopylae fame. The novel intermixes a series of "notes on a thesis", Ben's background notes for a thesis he's composing. The thesis explores the dark side of the Spartans, which contrasts with the more inspirational side of the Spartans as reflected in the story of the Battle of Thermopylae and as fictionalized by, for example, Steven Pressfield in [Gates of Fire], and the thesis notes are actually very interesting in and of themselves and not just in how they advance the storyline.
As the dig progresses, Ben ingratiates himself into the dig team and strange things start to happen. Modern as well as ancient Greece are well explored in The Hidden, but I found the activities of the dig team (I won't say more to avoid spoiling) to be a bit unmotivated and muddled. Still, the exploration of the darker side the Spartans was interesting to contrast with books and movies that celebrate them ([Gates of Fire] and the movie 300). I found the front half of the book to be much more enjoyable and convincing than the latter half, but overall it's an interesting read.