The Apocalypse Codex, by Charles Stross

The Apocalypse Codex is the fourth in the Laundry series of books by Charles Stross, and every bit as good as the first three, though not as humorous (or perhaps I am getting used to his wry take on summoning demon-spawn from the dripping maw of ichor that is Human Resources).  In this latest outing, Bob Howard (Robert E. Howard, the author of the Conan novels, was a great friend of H. P. Lovecraft, whose elder gods are the ultimate evil of this series), junior computational demonologist for the Laundry looks into a born-again evangelical intent upon hastening the Second Coming..but of what he has no clue. 

Bob does.

OK, no more hints.  Well, except that this assignment is kind of a training exercise for Bob, with all the frightening implications of that statement.

Read the book;  if you haven’t read the others read them first:

The Atrocity Archives

The Jennifer Morgue

The Fuller Memorandum

These links take you to the Amazon page for the paperbacks;  all are available as audiobooks, which I also enjoyed, but are pricier.  Kindle versions of all are available.