The Wandering Poisoner - The Toxicology Trilogy, #1

Holt Gnarl is a professional poisoner, wandering from master to master, perfecting his deadly trade. Despite his skill with toxins and antidotes, his heart has been poisoned against love, and for that he has no cure. He wants nothing more than to collect his fee and move on to the next job, with no strings attached. But when the Duke of Midair's heir is poisoned, Holt must head north to find a rare flower that is the only antidote.
Besides the brutal conditions, highwaymen, and renegade mercenaries, he rides into a plague sweeping over the countryside. Even worse, the duke's daughter, Riva, has followed him, set on seeing his task completed. To keep her alive, he must teach Riva what he knows. Showing her the formulas is easy, but ignoring his growing attraction to her is not. As they struggle to create a remedy for the plague, he discovers an ancient sickness that destroyed the old empire and is on the verge of spreading again. Will Holt save the kingdom and its heir, or will opening his poisoned heart to Riva doom them and leave the kingdom in ruin?