Drinking Midnight Wine — Simon R. Green

Toby Dexter is a slave to his own daily grind -- nine to five at the local bookstore. One evening he gets a reprieve in the form of a beautiful woman riding the same train. A woman he follows through a rainstorm. A Woman who opens a door in a wall that wasn't there a moment before...

So he follows her through the door, where a serpent stirs awake and an angel falls....

Toby has entered Mysterie....