Spark of Desire. Sorcery and Desire, #2

Six years ago, Spark's lover Komyau volunteered for an experimental process that gave him sorcerous powers -- a process the Convocation of Sorcerers was certain was safe, and went on to use on thousands more individuals in the intervening years. When Komyau becomes gravely ill, Spark contacts Thistle, the individual who invented that process, to save his life. Spark never expected her lover to fall for Thistle, or that Thistle and his husband, Frost, might be open to a polyamorous relationship.
For his part, Frost never realized that his best friend already was polyamorous. But that should not matter: Spark is a centaur and not attracted to elfs like Frost. Right? But if sorcerers can be mistaken about simple matters like attraction, they can make mistakes with far graver consequences -- and perhaps that experimental process is not so safe as everyone thought ... This novel is a standalone polyamorous fantasy romance, with M/M and M/F pairings.
Contains consensual transformation and intercourse with centaurs.