"at" sounds like she is going to be there studying or reading for some time. She might even be reading seated on the stairs outside the library. "in" sounds like she is just inside, maybe to meet a friend to go to lunch or to deliver a package or to pick up a book and leave right away
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Answer:
"at" sounds like she is going to be there studying or reading for some time. She might even be reading seated on the stairs outside the library. "in" sounds like she is just inside, maybe to meet a friend to go to lunch or to deliver a package or to pick up a book and leave right away