Existing non-fiction authors, feel free to promote here~
I expect a lot of people are going to say history...but I would probably write about science. Biology/Zoology, specifically.
I've always loved reading about plants and animals, and I appreciate that it's a relatively accessible branch of science: anyone of any age/from any background can start learning without any prerequisites; it's 90% observation.
Not like chemistry (my major...) or physics, where you have to know things about math and the discoveries of earlier scientists before you can really start to grasp the foundational concepts. The foundational concept of biology is "look at creature...and think about what you see".