Wolf's Rogue

Werewolf billionaire Zach knows only one thing—he and Stella, an assassin from an opposing pack, were meant to be together. But they've slept together and their mating bond didn't take, so he's condemned himself to be alone, whereas she's gone back to her world of being a killer for hire, until fate intervenes.
Her pack needs his help, and access to his money, which he freely gives her at the cost of including him in on her plans. He knows he and Stella were meant to be mates, and he'd visit hell with her to prove it, which is a good thing—because the creatures from other Realms that they're fighting just might take them all there.
This standalone book continues the Wardens of the Other Worlds series, which tells the stories of beloved side characters from the Prince of the Other Worlds books.