Masterful Lord of Thessaly by Rachael Slate

Hidden in plain sight
Lapith noblewoman Delia has been leading two lives. One, as the dutiful wife of the centaur Lord Hector. The other, a fierce Amazon warrior tasked with rescuing nymphs from a hellish fate of slavery. She’s just steps away from the largest rescue mission yet, and there’s only one thing standing in her way—her husband.

Unraveling her darkest secrets
As the heir to King Cheiron, Hector has borne the weight of an empire on his shoulders ever since his birth. With each passing year, Cheiron’s subjects look to him as their new leader. But he can’t rule a Kingdom when he can’t even solve the mystery in his own castle. The woman posing as his mate isn’t his wife, and the female avoiding him all these years is the one he can’t resist.

A love too late
When Delia disobeys the Amazon Queen to save Hector’s life, they are banished to Lapith lands—enemy territory. Forced to work together, they must ensure the survival of their races, and their hearts, before the coming war draws a line between them that neither will be able to cross.