Jezebel's Daughter by Wilkie Collins, Fiction Paperback - 2004

From the publisher

And she thinks that lunatics can be cured by kindness; and springs a man called Jack Straw from Bedlam to foster his sanity in her own household. . . . Meanwhile, in Frankfurt, Fritz Keller, son of one of the other partners, has fallen in love with Minna Fontaine, daughter of a sinister widow whose husband made a lifetime study of poisons. But Keller's father disapproves of the girls's mother, Madame Fontaine. Rightly so: the woman's constantly in debt. He refuses to alllow the kids to marry. Madame Fontaine is not a woman you'd want to get on the bad side of. She owns a chest of poisons and their antidotes which her husband intended to be destroyed on his death. She doses father Keller with a slow-acting poison and to win his goodwill revives him with the antidote. She becomes his nurse and then his housekeeper, having assured him, falsely, that she is no longer in debt. Keller withdraws his objections to the marriage of Fritz and Minna. Then Mrs. Wagner comes to Frankfurt on business accompanied by addled Jack Straw, who is much improved by her kindness and devoted to her. He is immediately recognized by Madame Fontaine as "Hans Grimm," mentally damaged by being accidentally poisoned in her husband's laboratory years before. . . .