Geographically

If you want to organize your books geographically, I suppose there are two basic versions: By the author's country of origin, or by the country in which the book is set. Both face the same problem: neither authors nor fictional characters necessarily tend to be born, grow up, work and die in one country only. But that's the challenge, isn't it?

Of course, you could have one shelf for the single-country authors or books, one for the two-countries ones, and so on and so on upwards to the true globetrotters. You could have all kinds of sophisticated systems which cross-reference authors' birthplaces, residences and settings. You could end up drawing three- or four-dimensional maps with pins and lines all over to keep track. Maybe, just maybe, you should have a bookshelf shaped like a globe.

If I were to do this, I'd probably go for ordering by the countries in which the books are set. With suborders for London, Värmland, Mississippi/N'awlins, et cetera, of course. I'd probably have to draw a line somewhere along the lines of one-country, two-countries, three-countries, up to five or so, and then just "books set in more than five countries".

And then, of course, there's the question of: what is a country?
Map from Dan Meth's Popculturalcharts.