> why would you suddenly become interested just because some percentage of the general public is?
Maybe the author has something to do with it? :) People like good books, and good books make dull topics seem interesting.
A good author can make a dry topic really interesting. "The Other God" for instance--how many people are going to be interested in a book about the history of dualist religion? But it turns out to be a real page-turner.
A good author can apply their skills in a surprising area and create a very interesting book. For instance, "The Cleanest Race" isn't just about North Korea, it's also about doing comparative analysis on propaganda. To me, this doesn't make the topic narrower, it makes it broader. People like me who aren't interested in comparative analysis find out it's an interesting tool when applied to something I'm interested in, and I imagine if you were the other way around you'd enjoy the book too.
I found out about "The Rise of Christianity" from a Catholic friend of mine, and now it's one of my favorite books, despite not being a Christian. The author took a pretty dry topic (the early history of Christianity) and made it interesting by applying modern sociology and anthropology to it. That he writes well and comes to interesting conclusions makes it a good book.