Bound Angel by Felicity Heaton

Second in command of the most fearsome legion of fallen angels in Hell, Rook only cares about one thing—seizing control of the First Battalion and moving one step closer to the Devil. But a dark angel of Heaven keeps seeking him out, insisting a witch needs his help, and one glance at an image of the ethereal silver-haired beauty stirs powerful emotions long forgotten and rouses a deep need he cannot deny or understand—one that commands him to risk everything and save her.
Isadora has been a fool. Millennia alone, enduring an eternal life created by the bond she shares with the angel she lost, has taken its toll on her. Desperate to see a familiar face, she followed rumours to Paris and placed her trust in a group of witches, only to be betrayed once again. Now, they want the spell that has made her immortal and she’s close to breaking… until a formidable warrior with crimson wings comes to her rescue, awakening old feelings never forgotten in her shattered heart.
With the mounting threat of witches and an enemy of old in danger of tearing them apart, can Rook untangle the truth about the past he can’t remember and can Isadora convince her demonic angel the forever he desires is the one he promised her?