'I Married A Dead Man' by Cornell Woolrich
Woolrich wrote under the pen name of William Irish and is credited by that name in the movie credits.
The book is better than the movie except for the ending, which is completely different.
The family name is Hazzard, not Harkness.
Helen/Patrice is much younger in the book, about 19 or 20.
Steve doesn't turn up until the baby is over a year old. His blackmail of Patrice takes place over a longer period of time.