Interestingly, it could be both.
Parts of it are almost certainly historical, especially the New Testament. Historians are fairly confident that they really are letters written around 100AD from one person to another.
Now, is everything that is said in those letters "truth"? That's the tricky part.
But its just like reading, say, one of Thomas Jefferson's letters. Its certainly non-fiction. Thomas Jefferson definitely wrote it. Was he wrong with what he said? Maybe, maybe not. But its not fiction.
The Old Testament is more interesting. Various oral histories, stories, myths, etc, that were passed down from generations and finally codified. Sprinkle in some poetry, some songwriting, some genealogy...whether or not its "truth" its still of interesting cultural significance.