It's no secret, at least to my philosophy students, that my favorite atheist is Friedrich Nietzsche. Were I an atheist (I am not) my "Bible" would be Thus Spoke Zarathustra
, as it was for Jack London and Eugene O'Neill.
Here's another nice little article
on Nietzsche's enduring cultural influence, generated out of U-Wisconsin historian Jennifer Ratner-Rosenhagen's American Nietzsche: A History of an Icon and His Ideas.