The Time Breaker - The Guardians of Time, #3

It has been five months and Emilia has still not returned to Ithaca. Seb, floundering without her, finally decides it's time to track her down and convince her to come home. But when he finds out just how badly he's hurt her, he begins to doubt she will ever trust him again. Completely obsessed with finding David's murderer, Emilia has neglected her friends, her health and her mental wellbeing in pursuit of her need for answers.
When the answers don't come and Seb eventually finds her, she reluctantly agrees to come home. But nothing is the same anymore. Damaged by what has passed between them, Emilia is determined to purge her feelings for Seb and sets about keeping him at an emotional distance. Seb desperately tries to break down her barriers, but before he has a chance to convince her he's serious, David's killer reveals himself.
and his plan just might tear the world, and Seb and Emilia, apart for good.