It's a four-book series, so far—there's a kind of looming doom thing going over the later books that hasn't come close to happening by the end of the fourth book, The Apocalypse Codex. Word is that he's planning on a 9-book series with the Big Event (called Case Nightmare Green in the books) towards the late middle.
Anyway, nothing much to say about these in particular. If you like thrillers, or spy-fi, or horror, they're worth a shot. I found them quite useful as an exercise inducement—always nice to have something compelling to bribe yourself with to get one's pushups accomplished.