Ooh ... gonna think on this one for a moment. Hmm ... At that point for me it almost becomes "Favorite Non-fiction book" which I'd have to point at a religious text, because I am devoutly Christian.

But I'm fairly sure that's not what you were going for. I'm going to stick with "Non-Fiction" because a lot of what's Sci-Fi and what isn't is kind of nebulous (Jurassic Park for example is definitely Sci-Fi but was not shelved as such by author request).

So I guess I'd have to say something like Bill Bryson's A Short History of Nearly Everything, David Willets' The Pinch, or maybe those Horrible Histories books I loved when I was younger.

I will give a special shoutout to Korman's YA books though, since they're technically not fantasy.