Fletcher Pratt 
The Blue Star [EPUB ebook] 

The Blue Star 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. Witchcraft is hereditary but the ability to use it can be held by only one member of a family line at a time, being passed from mother to daughter at the daughter's loss of virginity. The daughter's lover then gains possession of her magical talisman, a jewel known as a "blue star", which enables him to read the mind of anyone he looks in the eye. The catch is that he retains access to this power only so long as he keeps faith with his witch lover…