Story is in a medievalish setting and starts with a woman who has been falsely accused of being a witch being taken by 2 men to be sacrificed to a dragon.
She ends up befriending the (male) dragon, who is able to transform into various forms, including human, and can speak only the language of whatever form he is in. e.g. can speak English in human form, but can't in dragon form.
He needs to return to his dragon form every day at sunrise or he dies.
Contact with iron or steel causes him pain and prevents him from transforming. Near the end of the book, he's been put in handcuffs by one of the two men from the start and the woman has to get the assistance of the other one (village blacksmith?) to remove them before sunrise, which they manage to do just in time.
I think I read it maybe 10 years ago.
I also remember it was a very short book, probably only about 100 pages.