Its more a merger of museum and library, and you call them artifacts not books.
Think of plat maps and genealogical records from centuries ago. As a percentage of library artifacts, they'll increase over time.
Plenty of people will be "into" converting popular books. Then a smaller subset are into nation wide distributed but unpopular books. Then books not written in the local language or current language. Finally you end up with artifacts that are valuable to researchers but might only be read every century or so. Consider small town city council meeting records. Or church records of baptisms and funerals (and burials).
Aside from technological or economic collapse (good thing those two have never historically happened and therefore can never happen in the future...) the library of 2030 will be a lot like a museum is today, except perhaps more hands on. So you can walk into the genealogical room and its basically unchanged from 2015. Teen fiction, well, maybe that will be gone entirely. The concept of special collections or rare books will be eliminated because the only remaining books will either be special or rare.
What to do with all the space and salary is interesting. My suspicion is the days of 90%+ of American kids living less than 15 minutes walking distance from a public library are about to come to an end. I live in the county seat and have the largest public library, and probably every library in the county will eventually close and send all their genealogical artifacts to our formerly city now county library. We already have a small county historical society museum and I would not be surprised to see them merge with "the" county library.
Probably not a good decade to try to get a library science degree and become a librarian.
Interestingly there is a subculture that refuses to read off screens, even here, on an online on-screen discussion site, and that combined with very rapid printers and print on demand and ordering online means that bizarrely enough there might be more paper books in 2030 than in 2015, its just that cities won't pile so many up in one building. Much like the decline of public wells as a source of urban drinking water doesn't mean we've evolved past water drinking, it just means we have more faucets than mouths now, including faucets in our homes, so a neighborhood sharing a well seems weird and is basically culturally dead. Likewise in 2030 the idea of sharing a pile of books might feel icky, like a neighborhood sharing underwear or shoes does today.
