The Paper Magician by Charlie N Holmberg (2014) 47North

A woman, Ceony, is apprenticed to a paper magician and must save him when another magician literally steals his heart for dark magical purposes.  She follows the woman (Lira, his former wife) who stole the heart on a magically enhanced glider to the seashore.  There Lira uses blood magic to put Ceony into the heart and she must go through all four chambers to find her way out.  She sees some of his memories and learns a magic ritual from him.  When she gets out, she performs the ritual, freezing Lira and manages to take the heart back to her teacher.