With every book you write–fiction or nonfiction–facts matter. Nothing pulls a reader out of a story faster than the feeling that something is terribly wrong or just not true.
Sure you need to do research when writing historical or police procedural, but to me those usually aren’t where authors miss. Those are things most authors know they need to research. The misses are in the little things.
Have you described someone as not being able to eat dairy because of their Celiac Disease? Miss. Celiacs can eat dairy.
Have you described the mountainous landscape of Minnesota? Miss. Even if Minnesota technically has mountains, no one thinks of it as mountainous.
Writing a book is far more than just sitting down and creating, it’s also researching and understanding who your characters are based on certain real-life facts. If you give your character a disease of any sort you need to research and understand what that disease is.
If you are setting your book in a world outside of the one you’re familiar with, you need to study maps, pictures and read about the place to make sure your Victorian house belongs there.
Facts matter because the minute your reader finds something you’ve written unbelievable they will find the rest of the story equally unbelievable.