Mesmerized - Federation Chronicles, #4

They call him the Shadow. Andrei Solace delivers death on silent feet as an assassin for the Phantom Corps. The perfect weapon in the fight against the Imperium, he's sent to the same 'Verse he left behind as a youth. Piper Roundtree is a mercenary prepared for anything-except for being rescued from Imperial lackeys by the man she hasn't laid eyes on in eleven years. While she remembers every mesmerizing sensation he gave her, she thought they were through.
And against the backdrop of an impending war, their passion is reignited. The girl Andrei once loved is now a fierce woman who can pilot a ship through canyons at high speeds, and drive him wild with equal abandon. As much as Andrei tries to protect her from the darkness inside him, Piper admires his skills and the code he lives by. Now the lovers must join forces-in more ways than one-to save the Known Universe before the enemy hurls it into irreversible chaos.