Blue Star by Fletcher Pratt

The novel is set in a parallel world in which the existence of psychic powers has permitted the development of witchcraft into a science; in contrast, the physical sciences have languished, resulting in a modern culture reminiscent of our eighteenth century. The protagonists are Lalette Asterhax, a hereditary witch, and Rodvard Bergelin, an ordinary government clerk who has been recruited into the radical conspiracy of the Sons of the New Day. Rodvard, though attracted to the daughter of a baron, is commanded by his superiors to seduce Lalette instead to gain the use of her blue star in the furtherance of their revolutionary aims. The witch is no more truly enamored of him than he is of her, but both fall in with the scheme for their own reasons, unaware of how much they are simply pawns in the larger scheme of things.