Also worth reading: http://www.paulgraham.com/stuff.html
Books piling up exert pressure to be read, but simply succumbing to the pressure to read them all might result in falling victim to what Einstein talked about:
"Any man who reads too much and uses his own brain too little falls into lazy habits of thinking."
I don't feel books piling up exerts any pressure to read them all. I have a lot of books I don't feel compelled to read, but find it beneficial to have ready for me to use whenever: reference books.
I also have a lot of books I already have read, and have no plans to reread, but will keep regardless, in case I ever need to refer to them to refresh my memory. As an example, I still occasionally will find reason to refer back to a book I read twenty years ago, to confirm something so that I can confidently cite it in a discussion I am having.