I'm shelving this one under 'nonfiction' and 'history' since that was the intent of the writer at the time of its writing. And it was considered as such by his contemporaries. Of course, it's more fiction than fact.
I admit to getting very excited when I finally got to the Arthur section. Nevertheless, it was fascinating to read about some of the characters that inspired Shakespeare and to recognize Geoffrey's heavy borrowing from Nennius's founding myth of Britain. I still want to read Gildas and Bede, but I'll leave them for some other time while I read more Arthuriana.