I see that for people who have books, it is harder to reduce the number of books, than any other possession. People are more likely to die with a big book collection than anything comparable. The exceptions to this statement are women's shoes, coin/stamp collections, family pictures and bonsai.
I think the reason is that books, because they exist only as a part of the mind, are a more central part of identity than most other things. Part of the mind is a key element. While books are good as weights to flatten a wet drawing, as door stops and to use as a booster seat, they really have no function except as they have been read, known about or in some other way processed by the mind.
It is this mind connection of books that helps them be so integral to identity. The presence of books, books in a book collection is a way to make visible a uniquely human attribute: the human mind. For people who have an active mind... this is an important element of identity.