Sweet Little Spies by Lexi Blake

Since he was a kid, Aidan O’Donnell has known two things about the world. Tristan is his best friend, and Carys is the love of his life. Sharing her with Tristan was oddly easy. They both loved her deeply, and they never cared what anyone else thought. They were a team and everything was wonderful. Until the day it ended.

Carys Taggart has spent the last year and a half of her life living a lie. A lie Tristan forced on them all. She understands that it was meant to protect her and Aidan, but lately when Tristan says he doesn’t love her, it feels more like the truth. The wedding she’s dreamed of has been put off far longer than he promised. When he asks her and Aidan for another delay, she’s ready to move on without him.
Tristan Dean-Miles has a good plan and the best of intentions. Go undercover as a ruthless arms dealer so he can find a deadly bombmaker at the top of the agency’s wanted list. It might be taking longer than expected, but he’s so close he can taste it. Unfortunately, getting this close meant getting in way too deep. He knows he will succeed, but if he can’t convince the love of his life and his best friend that he’s worth the wait, his victory will cost him everything.