Sawyer. Earth Resistance, #2

There's no time for love in the apocalypse... Fighting to survive in the brutal aftermath of the alien invasion, Ben Sawyer knows he's dangerous. During his career as an undercover cop, he lied and betrayed those who trusted him. Now, the only thing that keeps him going is his ability to lockdown his emotions and put the job first. Relentlessly, he fights with the rest of his team for England's few remaining human survivors, all while ignoring the growing void in his heart.
Until he rescues feisty weapons engineer Dr. Julia Simmons. Julia had her life all mapped out as one of the British Government's elite experimental scientists. Now, she's pissed. The Chittrix invasion took everything from her and she's out for payback. Working on her latest prototype, the last thing she needs is any kind of emotional distraction.especially the rugged intense type - like Sawyer. But in the confines of the underground base, fierce attraction explodes between them even as an unexpected alien mutation threatens the previously impenetrable bunker.
With time running out, Julia and Sawyer must work together to survive the dangers closing in on them before terrible secrets from his past threaten to drive them apart. Sawyer is the second book in the Earth Resistance Series. Each book is a standalone romance, which follows an overarching story. They can be read in isolation but for maximum enjoyment are best read in sequence.