I don't think either of these covers captures the feel of the books themselves: Conjure Wife is another example of an older book republished during the height of the Gothic romance fad (this edition is from '68, art by Jeff Jones). Its current edition has a pretty foxy Goth chick on its cover, reminiscent of the loverly Eva Green. Berkley's Our Lady of Darkness has an odd psychedelic tinge to it, dated even by 1977 standards; fortunately Amazon has it listed as being back in print this fall.
...sometimes it wasn't clear whether it was a real woman, or a goddess, or some sort of metaphorical entity that de Castries was talking about. "She is all merciless night animal," he would say... "She knows the cities' secrets and their secret weaknesses, their ponderous rhythms and dark songs. And she herself is secret as their shadows. She is my Queen of Night, Our Lady of Darkness."