Medieval Underpants and Other Blunders: A Writer's (and Editor's) Guide to Keeping Historical Fiction Free of Common Anachronisms, Errors, and Myths by Susanne Alleyn is the single most useful book I've read on the craft of historical fiction. It's presented in a compulsively readable style that made it hard to put down. Riveting and beautifully logical, the book's mantra is never to assume you know something. But don't take my word for it. Let Susanne Alleyn tell you why. I picked this up after I read a highly entertaining excerpt about misconceptions of the Terror of the French Revolution that was also full of good historical research methods. That's her favorite topic, but she's just as passionate about all the other details.
The most fun sections are the examples she takes from real books and movies in order to tear them down. Who can resist laughing at the mistakes of other people? Of course, after you're done laughing, if you're writing historical fiction, you're left with the vertiginous realization that your current draft commits many of the same errors. At least you'll have read this book and learned how to correct such blunders before an editor sees it!
This book is also highly recommended for non-writers who like to read about the tasty morsels of history that don't get covered in straight history books, such as, how women relieved themselves before the advent of public restrooms and why medieval teeth weren't rotten.