Back in 2008 I read a trilogy of fantasy books. The series was about a man who had slain the last dragon that had been seen in the land and his wife, who was a low powered witch. The first book starts with the main character being called by the king or some higher lord to kill a new dragon that has taken over a gold mine. Dragons are fully sentient and powerful magic users. This new dragon is a very old black dragon and ends up getting into a kind of relationship with the wife. At the end of the first book the dragon turns her into a dragon but she ends up changing her mind and becoming a human again to be with her husband.
The next two books were about the man, wife, and the dragon fighting a demon I think. Another thing, dragons like gold because they can sing to it and it resonates really well. Dragons also don't breath fire, they spray some kind of acid, or maybe a highly oxidizing fluid.