The Bedlam Stacks by Natasha Pulley

From the author of The Watchmaker of Filigree Street

In uncharted Peru, the holy town of Bedlam stands at the edge of a mysterious forest. Deep within are cinchona trees, whose bark yields the only known treatment for malaria. In 1859, across the Pacific, India is ravaged by the disease.

In desperation, the India Office dispatches the injured expeditionary Merrick Tremayne to Bedlam, under orders to return with cinchona cuttings. But there he meets Raphael, an enigmatic priest who is the key to a secret which will prove more valuable than they could ever have imagined.