The Door through Washington Square Mass Market Paperback – August 1, 1998 by Elaine Bergstrom 
 
Dierdre MacCallum is summoned to her great-grandmum Bridget's house to settle her affairs. She opens the sun room's french doors to the sunlight-- and finds a doorway to the past, seventy-two years ago.
There she will meet great-grandmum as a young lady. She will find love with a man destined to die before she was born. And she will find danger as she uncovers Bridget's secret--her involvement with the infamous Alistair Crowley, whose dabblings in the dark powers promise destruction for the MacCallum clan.
Now Dierdre must set things right, and rescue both her family, and her one true love...
 
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