I've never written historical fiction though I love reading it. I too big a coward to tackle historical fiction. Readers will spot those mistakes and let you know or worse let the whole world know in a review. But since the epic fantasy I write is set in a medieval setting, I try to insert some realism into the cultures I create.
A book I keep on my desk, What Kings Ate and Wizards Drank by Krista D. Ball, covers loads of interesting facts. For example, how long does it take to make a stew over an open fire? Two hours to get that meat tender. What vegetables and roots could travelers find in the woods? Does the hero carry a thick, metal cooking pot and the big knife needed to field dress game? Great answers in this book?
Getting enough to eat was a real concern during medieval times and malnutrition often made people vulnerable to other illnesses and diseases. Of course there was the scary bit one, the plague, that wiped out a third of Europe. And it's still around today though not nearly as frightening. Read about modern outbreaks. Of course in a fantasy novel magic is always an option for health woes but I can add some depth to my society by also including real methods that might be used in a medieval society. The History Learning Site has lots of interesting information on medieval life. Another resource I use is a book of herbal remedies, many that are served in teas. Doesn't every medieval book need to serve up a little tea now and then.
Did you ever read a historical novel and spot a glaring error? How about a fantasy novel or science fiction novel where you just didn't buy into a piece of the story? Ever read a novel like I did last week that was so confusing in the world building I couldn't get interested? Had you heard about the recent cases of plague in Colorado?
