There were actually no real overtly supernatural elements. It was just eery, and very nicely done. 
I guess l'm just stubborn about it. Like I said, I'd like there to be an explanation but I have a hard time explaining away everything in Dunwich. Point Lookout makes it even more overt by providing a history for the book.
The hallucination I can put down to a pocket of gas. The poltergeist phenomena I can explain by noting that the building is in rough shape and settling. The madness of the Dunwich employee I can explain by exposure to the sonic borer. The altar/statue in the basemant? Carved by the insane employee, maybe?
Jamie's rapid ghoulification/madness once he found the book, which he describes as warm to the touch? Maybe the book is (was?) emitting high radiation?
This stuff can be explained, though the conclusions are dependent on an unlikely confluence of events. I really think the writers meant for us to accept it as supernatural, though. Go read the holofiles and computer entries on the WIKI & maybe you'll see what I mean.
Point Lookout pretty much underscores the supernatural angle, though. I have absolutely no explanation as to why the book would burst into flames like that.
BTW, this is just for discussion, you know. I don't have an axe to grind about Fallout 3; I don't think the supernatural fits the setting but I'm not incensed by it or anything. If you think I'm one of those tiresome purist types, I can prove I'm not -- Mothership Zeta is my favorite F3 DLC.
