The Murder in the Tower — Jean Plaidy

The beautiful Frances, Countess of Essex, would let nothing stand in the way of her desires. And now she desired the handsome Robert Carr, favorite of the doting King. Not letting her own marriage deter her, Frances pursues the young friend of kings and poets, and when her advances are met with cold disdain, she aligns herself with the world of spell-makers and poisoners. If her own natural charms cannot win her love (and rid her of an unwanted husband) then she will not scorn to seek her wishes from the dark underworld of the supernatural. All the kingdom and heaven itself would not withstand her wiles.