I've heard it said that writers need big libraries. Do they? No.
What writers need is access to information. Fifty years ago that meant having a well-stocked public library nearby or your own one at home. In the 21st century, you need a computer, an internet connection and several bookmarked pages such as Project Gutenburg.
While I use the internet for some research I have a big library, over 500 books at the last count. That said, I LOVE collecting books, which some people don't. I don't keep hold of books that turn me off completely, and there aren't many that do, but the rest are kept on one of my shelves.
I have one 15 foot shelf on my bedroom wall and will put another one up at some future point so I can put some of the double stacked books on my three free-standing shelves on it. That will leave me plenty of space to add yet more books, a lot of which I get from second hand shops these days.
How much of this is needed? Not much. In fact, I have only read just over half of them. Some of the books I bought because they're old, some to read and some to have just in case I need to refer to them. The largest part of my fiction collection is taken up with SF and SF/F comedy with Terry Pratchet having a shelf to himself. Tom Holt is rapidly getting to that point too.
So why so many? It's my anti-library. Library books are popularly misconceived as places where every book will prove useful to some one. If that isn't the case, then it's thrown out. My library contains books that I will probably never read, but have in case I need them at some future point, or just to pad out the huge library I'll have when I earn squillions with my writing (we all need dreams).
No writer NEEDS a library, and most don't have the space for one, but we all need some books, such as a dictionary and thesaurus, and a method of obtaining the information we do need. For that, in our day and age, an internet connection is just as good as a library.