Sacred Fire by Tanai Walker

Tinsley Swann is cursed to change into a beast for seven days, every seven years. She keeps her distance from the world, and has more of a relationship with the antique erotic postcards she collects. With the time of her transformation approaching, she finds herself torn between two women. One woman is Sandra, Tinsley’s new boss, and the two are having an affair. Sandra glimpses her transformation and is kind, not frightened. The other woman, Leda, bears a striking resemblance to one of Tinsley’s turn of the century postcards, and she becomes obsessed with the young woman. Tinsley must choose between these two women and ultimately two factions, one that will save the world, the other with plans to destroy it.