It sounds a bit like Jonathan Strange & Mr Norrell by Susanna Clarke.
It follows the two title characters who are initially working together but later feud. There is some time given to trying to track/capture fairies.
Wiki mentions a character using a mirror to move between locations in the 3rd act/part
Jonathon Strange definitely sees candles burning inside people's heads as he goes a bit insane towards the end.
“May I ask you something?" Dr Greysteel nodded."Are you not afraid that it will go out?"
"What will go out?" asked Dr Greysteel.
"The candle," Strange gestured to Dr Greysteel's forehead. "The candle inside your head.”
(quote from https://www.goodreads.com/work/quotes/3921305-jonathan-strange-mr-norrell?page=2)