Under the Rose by Kathryn Nolan

She’s about to go undercover with her arch-nemesis…
From the outside, the gorgeous Freya Evandale seems like a quirky bookworm with a taco addiction. Most people would never guess she’s a proud drop-out of the FBI’s training academy and a highly-skilled computer nerd. These days, she’s happy as a private detective at Codex, specializing in stolen rare books.
That is until an old nemesis with an arrogant smirk and superhero shoulders ends up in her office.
Special Agent Sam Byrne is a tightly-wound workaholic with a secret…or ten. When a betrayal leaves him questioning his career with the Bureau, consulting for Codex seems like the perfect place to hide. But he certainly never expected to see Freya again. At the academy, their rivalry was legendary. And their sparring sessions resembled an actual battle, or worse… foreplay.
Now these enemies are forced to play nice as they go undercover to infiltrate a secret society. It’s a dangerous world of power, wealth, and shifting alliances. The higher the stakes, the harder it is to resist the true reason behind their rivalry. But surrendering to years of pent-up lust is a direct threat to their mission…and their safety.