Branded by Fire (Psy-Changeling #6) by Nalini Singh

Though DarkRiver sentinel Mercy is feeling the pressure to mate, she savagely resists when Riley Kincaid, a lieutenant from the SnowDancer pack, tries to possess her. The problem is not simply that he pushes her buttons; the problem is that he's a wolf, she's a cat, and they're both used to being on top.

But when a brilliant changeling researcher is kidnapped from DarkRiver territory, Mercy and Riley must work together to track the young man - before his shadowy captors decide he's no longer useful. Along the way, the two dominants may find that submitting to one another uncovers not just a deadly conspiracy, but a passion so raw that it'll leave them both branded by fire...