Cecelia isn't overwhelmingly distraught when her great-aunt dies. It's sad, of course, but they weren't especially close, and people die every day. She left everything to her niece Mariel, but Cecelia's mother is in another state, whereas Cecelia happens to be attending college much closer. It's a coincidence, but a convenient one; Mariel deputizes her to do most of the legwork for the estate.
It is while rummaging through the old house for things worth keeping that she finds the books.
The first circle she copies out on a lark, sure that it will do nothing. Instead, it nets her a fairy who's rather annoyed to have such a startled summoner with no specific task. She dismisses him, and thinks.
And studies the books.
She thinks of things she wants done. She summons fairies, and later angels. She tells her family about this, and proves it, and they do not sell the old house.
Eventually, she's confident enough in her bindings to summon a demon (she could have used one of the ones in the book, but all the ones for demons have gags, and that is not a handicap she wants to have to deal with).