Umberto Eco apparently gets this question on a regular basis, what with having a 30,000 volume library as of 1994 (I wonder how much it's grown in the past 14 years).
[For] people who possess a fairly sizable library (large enough in my case that someone entering our house can't help but notice it; actually, it takes up the whole place.), visitors enter and say, "What a lot of books! Have you read them all?" At first I thought that the question characterized only people who had scant familiarity with books . . . but there is more to it than that. I believe that, confronted by a vast array of books, anyone will be seized by the anguish of learning and will inevitably lapse into asking the question that expresses his torment and his remorse.
In the past I adopted a tone of contemptuous sarcasm. "I haven't read any of them; otherwise, why would I keep them here?" But this is a dangerous answer, because it invites the obvious follow-up: "And where do you put them after you've read them?" The best answer is the one always used by Roberto Leydi: "And more, dear sir, many more," which freezes the adversary and plunges him into a state of awed admiration. But I find it merciless and angst-generating. Now I have fallen back on the riposte: "No, these are the ones I have to read by the end of the month. I keep the others in my office," a reply that on the one hand suggests a sublime ergonomic strategy and on the other leads the visitor to hasten the moment of his departure.
-Umberto Eco, "How to Justify a Private Library" in A Passion for Books
as quoted here.