Daughter of Darkness by J.. Lowell

 I have finally got my hands on a decent copy of this book, the first novel I ever read in one day:

It is similar to, though not quite in the same league as Elizabeth by Jessica Hamilton. And annoyingly (for me at least), just like Elizabeth, it was written under a pen name.

J.R. Lowell is actually a pseudonym of the husband and wife writing team Jan K & Robert M Lowell.

As you can see from the pic below, I have a bit of a soft spot for this book and its various editions: