Never the Roses by Jennifer K. Lambert

The Dread Sorceress Oneira has retired. She’s exhausted from fighting the endless wars of kings and queens, and has long accepted that her death is near. Alone at last but for a few uninvited companions—a near-mythical wolf, a goddess’s avatar, and a feline that embodies magic itself—Oneira realizes that she’s bored. On a whim, or perhaps at the behest of fate, she makes an unlikely trip to the most extensive library in existence: the home of her most powerful rival, the sorcerer Stearanos.
By recklessly stealing a book from him, Oneira inadvertently initiates a forbidden correspondence. Taunting notes and clever retorts reveal a connection neither has found—nor could ever find—in any other.
But Oneira soon learns that Stearanos, bound to a vile king, is tasked with waging war on the queen she once served. A relationship with him is far too dangerous to pursue despite their mutual desire—and yet, Oneira can’t seem to stay away.

A bond with Stearanos could alight the long-extinct flame of life within her… or it could destroy her entirely.