Ghost Exile Book 2. Spoilers ahead for earlier works.
Caina decides to investigate the home of a dead Master Alchemist, since she has reason to suspect he's involved in the sorcerous plot she knows is going on. She finds quite a deal -- such as a magical ring, guards who unexpectedly catch her, and others who give her unexpected aid.
One of the latter offers to bring her into a daring raid. She recruits a strange locksmith for it, and they go to Callatas's Maze while he holds a celebration.
It involves mirrors, Caina's claiming to have a dead sister, mirrors with connections, an ability to do harm to an Immortal by punching him, two characters baiting each other over secrets, and more.