Medieval Underpants and Other Blunders: A Writer's (& Editor's) Guide to Keeping Historical Fiction Free of Common Anachronisms, Errors, & Myth, by Suzanne Alleyn
I got a huge kick out of this book, despite the fact that I am not in the intended audience at all. It's a fun book of hints for people who write (or want to write) historical fiction. I have no desire to write anything much besides blog posts, especially historical fiction, and I don't even enjoy reading historical fiction all that much, but that's often largely because of the mistakes that Alleyn warns against here.
Alleyn is witty and very funny as she warns against common mistakes like feeding your hero food he could not possibly have eaten, making someone pay three gold louis for a minor purchase, wrong usage of aristocratic titles (a minefield, admittedly), or giving your heroine underpants. My husband was made happy by the existence of an entire chapter devoted to correctly describing guns ("...the words 'pistol' and 'revolver' are NOT synonymous or interchangeable"). There are wonderful chapters that go into detail about issues like how often people used to take baths (as often as they could afford to, usually), why servants were necessary, and how very slow travel used to be.
My favorite part was the chapter warning against "feisty females." That is, you cannot pop a woman with 21st-century attitudes into a Worth gown and have her run around unchaperoned all the time/save everyone with her superior morals/otherwise act in a way that would have gotten a real woman in huge trouble, and still have a good book. This is a personal peeve of mine, so it was great.
There is lots of great detailed historical information in here even if you have no interest in writing historical fiction, and it's a fun vindicating read if you are regularly driven mad by anachronisms in books and movies.