Momentary as a Sound — M.L. Hamilton

Avery Nolan worried for her friend, Agent Graham Braddock, when he chose to stay behind in the Frostfell, but she never expected to see him show up at her door, asking for her help. As she feared, scientists have found a way into the Greensward. Avery and Duncan agree to accompany Braddock back to the land they swore never to visit again, so they can stop a war that will tear apart both worlds. However, Avery's father, Alberon, discovers his daughter's involvement and has plans of his own. Alberon isn't worried about preventing genocide as much as he's interested in getting to know the daughter that he'd lost so long ago.