Lone Wolf: An Orphan X by Gregg Hurwitz

Once a black ops government assassin known as Orphan X, Evan Smoak left the Program, went deep underground, and reinvented himself as someone who will go anywhere and risk everything to help the truly desperate who have nowhere else to turn. Since then, Evan has fought international crime syndicates and drug cartels, faced down the most powerful people in the world and even brought down a president. Now struggling with an unexpected personal crisis, Evan goes back to the very basics of his mission - and this time, the truly desperate is a little girl who wants him to find her missing dog.

Not his usual mission, and not one Evan embraces with enthusiasm, but this unlikely, tiny job quickly explodes into his biggest mission yet, one that finds him battered between twisted AI technocrat billionaires, a mysterious female assassin who seems a mirror of himself, and personal stakes so gut-wrenching he can scarcely make sense of them.

Evan's mission pushes him to his limit - he must find and take down the assassin known only as the Wolf, before she succeeds in completing her mission and killing the people who can identify her - a teenaged daughter of her last target, and Evan himself. Matched skill for skill, instinct for instinct, Evan must outwit an opponent who will literally stop at nothing if he is to survive.