Every few years, network theorist Valdis Krebs creates a fascinating map of the relationships among the top political books currently selling on Amazon. In past years, he notes, "we saw a divided nation in our book buying data. We saw then a distinct red cluster and a distinct blue cluster with very little holding them together in terms of cross-links or books in common."

But now, he's found something different going on. First, there are a lot more books in the middle, "purple" zone--meaning that they were of interest to book-buyers on the left and right. And second, a number of "old conservative" writers like George Will and Patrick Buchanan were positioned closer to the progressive audience than they were to the right, an effect that Krebs speculates may have something to do a split between old-cons and neo-cons. "Even Ron Paul's and Jesse Ventura's books link more with the blue than with the red," he notes.

What's going on here? Krebs suggests that this map might have something to do with Barack Obama's post-partisan tone, and John McCain's efforts to appeal to "purple America." And he concludes his post by asking, "Is the country moving from slightly right of center to slightly left of center?" Well, the polls show that is happening. But I think the answer behind the shifting relationships on his map is simpler. Current book-buying tastes seem to be neatly cleaving on whether the purchaser supports the Iraq War or not. Clustered on the right are titles like "Surrender is Not an Option" and "Moment of Truth in Iraq." There's a big gap between them and Scott McClellan's "What Happened" (which is popular across the map), and with books like Kevin Phillips' Bad Money or Cliff Schecter's The Real McCain. And the same is true for the Ron Paul and Jesse Ventura audiences--they may not be progressives, but they are definitely anti-war.