From The Diamond Contessa:

KENNETH BULMER is English, lives in Tunbridge Wells, Kent, and is one of the leading lights of British science fiction as fan, editor, writer and war-games master.  Author of many novels, some under pen-names, he has been in print continuously for over twenty years.  In America he is best known for a series of novels about parallel worlds which were written for Don Wollheim prior to the launching of DAW Books.  For a long time, he has been urged to return to that locale for DAW Books, and in The Diamond Contessa he has finally done so, bringing back the magic and marvels of many alternate worlds, of many doors to them, and of that arch-villain, the mysterious countess whose avaricious eye is always on our Earth.