A Stone Denied. Mórdha Stone Chronicles, #7

The stones and torc were hidden deep within the monastery. The four sections of The Book of Stones spread far and wide. The Heart Stone separated by two hundred years. They were supposed to be safe.For the last year, Stella Caldway watched her dearest friend, AJ Moore-Murphy, disappear into the fog more than once, traveling to the past with no certainty of returning. Though AJ came back, each time stronger than the last, Stella had no illusions about her own chance of survival had it been her that greeted the mist.
Nope. She's just fine living with her modern-day conveniences, and most importantly-a coffeehouse on every corner. Until one fateful day, when Stella should have followed her own advice and stayed in bed, the past returned-the stones once again refusing to be silenced. And Stella fell into the rabbit hole, kicking and tossing her cookies the entire way. Theodore Beckworth, the Viscount of Waverly, has found his dream profession as a spy in Hensley's network.
A position where he can finally display his full range of skills. When his current assignment puts him in the crosshairs of Gemini, his past returns, and he soon discovers his loyalties tested. On the run for their lives, two people will discover they are stronger together than apart.