For a while, years ago, I was on an "alternative history" fiction kick. Most of it is dreck, but one diamond in that rough was Orson Scott Card's Pastwatch: The Redemption of Christopher Columbus[1]. Without giving too much away, it's about a group of future historians that use time-travel to study past cultures, and discover that they can not only view past events, but influence them as well. It's got a couple of interesting semi-plausible what-ifs of different ways the Columbian Exchange might have worked out, under different parameters.

[1] http://amzn.to/2CvPY10