Harold Rabinowitz, "They Don't Call It a Mania for Nothing"
What it reveals, first, is that the questioner lacks imagination and may even be humor-challenged. It's a bit like asking a tall man, "How's the weather up there?" It may have been amusing once, but that once was a long, long time ago.
The second thing the question reveals about the person asking it is that this person lacks a passion for books, A Passion for Books being the title of the book from which the essay quoted above is found. Those who feel at all passionate about books never wonder why someone else shares that passion. They are more likely to be amazed that someone else does not.
As for the answer to that question, it being a yes or no question, there are just two basic answers. If the answer is yes, the owner of the library has read all those books, then maybe he or she really doesn't have enough books. Yet much can be said, especially when one has reached a certain point in life, for just rereading the books one has already enjoyed through a long lifetime. If a book is any good at all, reading it again and again is hardly a waste of time. Yet to the person asking the silly question, an affirmative answer may lead to yet another silly question, "If you've read them, why do you still have them?" To many people, including some who read a lot, a book is regarded as being much like facial tissue: When you're done with it, get rid of it.
Most likely, however, the person with a large library will not have read all those books. Some of those books may have been sitting on the shelf a long time waiting to be read. Some may never be read.
In truth, how this question is answered doesn't matter. It may not sound like it, but it is really a rhetorical question to which no answer is necessary. The person asking it is simply expressing amazement that anyone in his right mind would want to have so many books cluttering his home. Other people's passions can be difficult to comprehend.