Nan asked about the willingness of an agent to accept writers who cross genres.
I offer two reasons.
1. Think of those who will read your books. Most fiction readers stay within the lines of fiction and it’s also true with nonfiction readers.
2. When you sign with an agent you want to build your platform/identity. You build it with a focus on one genre and each time you write another book within your identified field, you add loyal readers.
Although I’m an exception in that I cross genres within the nonfiction area, I’m primarily identified with ghostwriting and collaboration. That’s still within the nonfiction area. A couple of years ago I published 3 cozy mysteries and they haven’t done particularly well—certainly not in comparison with nonfiction books under my own name. Readers don’t follow writers when they depart from their identifiable area.
Few writers cross the line successfully with both fiction and nonfiction. If you feel you want to widen your range, build your reputation in one genre. After you’re established you might try to expand. John Grisham and James Patterson have both published a nonfiction book within the past three years. They did well because of name recognition—but nothing like the sales of their novels.