Forbidden Access by Gemma Ford

They call her Thorn for a reason. Sure, she can be prickly, but she was also one of the best agents they had. Trained in undercover surveillance, she’s an expert at hiding her emotions, pretending to be someone else. Now, as a top operative for Blackthorn Security, she’s been handed a mission unlike any other. Protect a reclusive crypto billionaire from a vengeful arms dealer. The twist? She must pose as his wife.
Damian isn’t your typical tech nerd. With a shady past and a revolutionary cryptocurrency upgrade that could expose dark web criminals, he’s got a target on his back. He’s on a mission to set things right, but his life depends on the edgy private security operative assigned to keep him safe. But he never expected his bodyguard to be a woman, much less one who ignites a fire within him that he thought had long been extinguished.

This is a gripping, fake marriage, bodyguard romance with some steamy scenes and a guarenteed happily ever after.