Perfect Poison by Amanda Quick

Quick [Jayne Ann Krentz's pseudonym for historical romance] picks up the story of the founding of Jones and Jones - the Arcane Society's investigative arm and the relationship between Lucinda Bromley, whose talent is with plants and poisons, and Caleb Jones, a direct descendant of the founder, Sylvester Jones. Caleb has been told his whole adult life that he will go slowly mad like all the males of his family. Lucinda is notorious in her own right after several suspicious sucides in her immediate family and the death of her fiance by poison. When a rare plant from her conservatory goes missing and the poison made from it begins killing people, Lucinda helps Caleb track down the culprit. Caleb finds comfort in the daily routine of Lucinda's household and their personal relationship develops as the investigation leads them in some very dangerous directions.