What should you write about?

There are millions of possible subjects. Here's one way to choose.

“What made you decide to write about that?”

I get the question a lot. I suppose most non-fiction writers do, although the answer is often obvious. A medievalist writes about Charlemagne or Prester John. A chef writes about soufflé. A memoirist writes about himself. Writers are told to write what they know. Most non-fiction books involve just that.

It has…