I buy books very often for DD, sometimes new, sometimes from charity shos. I don't see them as something I should save for special occasions and buy them without considering them as being a special treat.
DD does have 100s of books. She has bookcases in her room and also uses the one in the living room. She also has books on her bed shelves, in the bathroom, in her playroom...
But we do regularly cull. Those finished with I recycle - I either take them into the school I work at for our book browser, in the past I have sent some to DD's school, some go to charity shops, so get passes to family or friends; they never get thrown.
To me buying books is NOT the same as buying toys. Mind, I have no problem with buying the odd toy even if not at a birthday or Christmas.
I don't use the library. Our local one is a mobile library which is not that regular and the selection of books is very limited, even more so for children. The other library is a drive away although close to school, but because of after school events it is still not convenient.
Mind you, DD is about to join a book group at that library called Chatterbox, which is once a month - she will read a book, come together to chat about it, do related activities and games, etc.