I've just encountered a tool (new to me) for checking the Amazon “also bought” lists that point back at your books: Yasiv.
I typed in the name of the first book in my first series under Search Category = Books, and was fascinated to see how my readership broke down.
The series, The Hounds of Annwn, is a contemporary fantasy involving a Virginia foxhunter who ends up in the fae otherworld leading the Wild Hunt. Now, as it happens, I spent several years as a semi-pro photographer following the Virginia hunting scene, so when I published To Carry the Horn, the first book in the series, I had a ready-made audience of foxhunting enthusiasts who already knew me from my photography.
Many of these people have never read fantasy (beyond, say Harry Potter) and bought the series out of horse-related interest. The people who were already followers of Rita Mae Brown's foxhunting mysteries, with their fantasy elements of talking animals, were especially susceptible. There weren't any other fantasy readers who bought print editions where I showed up in their “also-boughts.”
I'm a writer of Fantasy and Science Fiction, not a horse or hunting mystery writer, but I chose this first topic as a bit of a crossover to appeal to my built-in audience so that I wouldn't have to start from scratch to build a fantasy audience, figuring my next series would be a more conventional fantasy (which it is).
I already knew that many of the first series readers didn't do ebooks and would be responsible for most of my print sales. What I didn't fully realize until now was how little connection there was between my print and my ebook audience.