When I was in grade school, late 1970s/early 1980s, I read a book in which a girl was in a mansion-type house that had a library. In that library was a window seat where she would sit and read a diary. The main story was the diary.
Stump the Bookseller
New format, same book stumper solving.
Perhaps “The Velvet Room” by Zilpha Keatley Snyder?
I THINK THAT’S IT!!! THANK YOU SO MUCH!!!