Endings and Beginnings

I recently finished the final (hopefully) draft of the core rulebook for The Neurojack Chronicles. Barring any gaping flaws, I'm done with rewrites, and have already sent the text off to my layout man back in Washington.

Having completed the first book in the series, I am faced with the question of what to write next. I know I want to do 3 sourcebooks on the major countries occupying my world (Northern continent, Southern continent and equatorial regions) but the question is which one do I write about first.

I already have a head start on the Northern sourcebook, since I actually wrote it a couple years ago for an independent study project in my final year of college. Though there is an abundance of material there, the world of Neurojack has changed radically since then, and I good portion of the book needs rewriting. That being said, should I finish the Northern sourcebook, or should I venture into more uncharted waters?

I've always had the best idea of the North in my head. If there was a single country I'd play in, it would be the north. I know it's kinda bad to have a distinct preference for one place over another, but the lines that divide the north from its neighbors have been blurred considerably more than they were and the war between the nations has been eliminated altogether, meaning that the north is now not so different from the south or the equatorial regions. There are still differences, enough so that writing about the south or the equatorial regions is a very different endeavor than writing about the north.

Should I leave the mostly-completed northern sourcebook for later and write about another country that I haven't fleshed out as well, or should I finish (or update, rather) what I started before I move on?

I know some of you are thinking "see your first book through to completion before you start on the next", and I'm still doing that. I just need to find something else to start writing while I'm doing that, since the rest of the core rulebook is largely in the hands of my artist and layout people.