Yellow Jessamine (Neon Hemlock) by Caitlinn Starling

In Yellow Jessamine, shipping magnate Evelyn Perdanu controls the dying city of Delphinium with trade deals and secrets. But when mysterious sickness sparks death and obsession, all leading back to her, Evelyn's brittle existence is strained to breaking. She retreats to her estate, amidst paranoia and poisonous secrets, intent on rooting out this plague before it destroys everything she has built.