Does this ring true for 13 yr. olds?
I would say go with what the library looks like. Merge that with the smells and descriptive stuff. For example, it isn't a huge old massive thing with super high ceilings and thick built in wooden shelves. It sounds more cottagey to me. Yeh, new word, you know, cottagey!
Or it could even be Victorian if you wanted it to be. What country is it set in? You could really play that up if it is in say, Wales or France or Ireland, or really anywhere in Europe, whereas you could play up the differences coming together of the modern and the well-known if it is set in the US.
But more back to the actual question...13 yr. olds, even girls, are generally active. Running and skipping here and there, talking incessantly, if there are two or more of them. I see you have her with another and not alone. Now, there could be another reason for them to shush and tread lightly and all, other than just that they are entering a library. You could bring out the additional reason, like, I don't know...a relative who always took them there, or a librarian, or that a certain boy is usually there engrossed in a book. You could have them furiously whispering between book aisles and then practically bursting when they reach the outdoors again. Or they could find something oh so interesting and stunning in a book that shuts them up and scares them a bit, and when they leave, it would be the topic of conversation. A mystery clue?
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