The agent’s answer
Asked whether a nonfiction book can be a comp for a fiction manuscript, Ismita says it depends, and that if you do it you need to explain the connection. For example, if your literary fiction is essentially a fictionalized version of a memoir, mention that, or if your historical fiction covers an event a history book also covers, say it is a fictionalized version of that incident. Her caution is not to use a nonfiction title as your only comp. Moderator Sarah Fisk adds the caveat they give for any non-book comp, using video games as their example. It is fine only if it is not your only comp, and only if there is not a book that would comp just as well. Reaching past an obvious book comp can signal you have not read widely in your genre. Ismita agrees emphatically: there is always a comparable book, she guarantees it, because people have been writing for too long.