The borrowing of books and other uses of public libraries in the city (as study rooms, use of internet connection, reading of newspapers, organizing/attending conferences, etc.) can give a good overview of a city's literacy and cultural level.
RFIDs in public libraries' cards (I missed first week's RFID Topic deadline, so I slip it here...) can give an idea of which and how often books are borrowed, and by which type of person. This could help the city identify which society groups or city quarters need more reading promotion actions.
As for the other uses of public libraries, this could give an idea of which premises could reinforce the needs or how to improve the more demanded services (I guess this part would need more help from the public library staff to gather relevant information).