Too Guarded to Love (Project Viper #3) by Kristie Wolf:

HE THINKS HE’S TOO GUARDED TO LOVE
 
Army Captain Nic Romero fears one thing: consequences.
 
Losing an arm and a leg in battle because he trusted the wrong woman was brutal enough. Now, he has a second chance at his military career with Project VIPER, the classified program that transformed him from a wounded warrior into an elite fighting machine.
 
This time, his walls are as impenetrable as his bionic body. But a blazing encounter at the beach with a secret-keeping siren threatens his defenses.
 
His only focus should be protecting her from a deadly threat. Problem is, her safety isn’t the only hard part of this mission. He’s already experienced the thrill of her addictive heat. Another hit could trigger consequences more dangerous than the enemy.
 
SHE THINKS SHE CAN’T BE PART OF HIS WORLD
 
Dr. Olivia Darren never meant for her overprotective brother to find out she seduced his VIPER teammate. But thanks to the trouble tailing her, one night with playboy Nic Romero has morphed into a fever dream.
 
Now she’s entangled in a classified world of secrets and shadows with Nic assigned as her protector. His constant presence screams danger, his touch feels like a dare, and the powerful feeling that she’s destined to repair his wounded heart won’t retreat.
 
She doesn’t belong in his perilous world but try telling that to her heart that may have finally found where it belongs.