The other approach views a book collection less as a testimony to the past than as a repository for the future; it’s where you put the books you intend to read. “I like to keep something on my shelf for every mood that might strike,” said Marisa Bowe, a nonprofit consultant and an editor of “Gig: Americans Talk About Their Jobs.” At its most pragmatic, and with the aid of technology, this attitude can be breathtakingly ruthless. Lisa Palac, a freelance writer, and Andrew Rice, a public relations executive, ultimately chose their beloved but snug house in Venice, Calif., over their library. “We’d been lugging these books around for years, and why?” Palac wrote in an e-mail message. Her husband said, “Do we really need to keep that copy of ‘The Scarlet Letter’ from college on hand? I can order up another copy online and have it tomorrow if I need it.” They kept only one carton of books apiece, donating the rest to a fund-raising bazaar for their son’s school.

Older people, curiously enough, seem to favor the less nostalgic approach. When you’re young and still constructing an identity, the physical emblems of your inner life appear more essential, and if you’re single, your bookshelves provide a way of advertising your discernment to potential mates. I’ve met readers who have jettisoned whole categories of titles — theology, say, or poststructuralist theory — that they once considered desperately important. Most of them express no regrets, although Nicholson Baker, who wrote an entire book protesting the “weeding” of books and periodicals from American libraries, still mourns the collection of science fiction paperbacks he discarded in his youth. “I’m not good at it,” Baker wrote in an e-mail message when asked about his own culling. “When I’m doing research, I buy lots of used, out-of-print books, preferably with under­lining and torn covers. I like watching them pile up on the stairs.”

For the most part, I’ve been pragmatic in my purging, and for years reference books were the most likely survivors. I needed them for work, for those occasions when I suddenly had to know at what age Faulkner published “Absalom, Absalom” (39) or the name of the Greek muse of lyric poetry (Euterpe). Now the Internet can tell me all that. Apart from the rare reference that’s worth reading in its own right, like David Thomson’s Biographical Dictionary of Film, these titles have been drifting away as the trust I’m willing to put in Wikipedia gradually equalizes with the faith I’ve invested in, say, Benet’s Reader’s Encyclopedia. (It doesn’t help that reference books tend to be shelf hogs.)

Nevertheless, most of the nonfiction I’ve kept consists of books I’ve already read and know I’m likely to refer to in my own writing. Richard Holmes’s biography of Coleridge has come in handy for more than one project, as has Carol J. Clover’s study of slasher films, “Men, Women, and Chainsaws.” In fiction, on the other hand, apart from a few choice favorites, the list is weighted toward classics I optimistically plan to get around to someday. Like John Irving, I hold one substantial unread Dickens novel (“Barnaby Rudge”) in reserve, for emergencies. This method has its pitfalls. The novelist Jonathan Franzen used to limit the unread books on his shelves to no more than 50 percent of the total. “The weight of those books seemed to represent a standing reproach to me of how little I was reading,” he said in a phone interview. “I want to be surrounded by books I love, although now sometimes I worry that it’s too familiar, what I see when I look around me, that it’s become a sort of narcissistic mirror.”

When it comes to novels, I’m probably too sanguine about what my future can accommodate. “Eventually the truth hits home,” Brian Drolet, a television producer in New York, told me. “As the actuarial tables advance, the number of books you’ve got time to read diminishes.” Dr. Johnson once said of second marriages that they represent the triumph of hope over experience. So, too, do my bookshelves. I have turned out to be less rational about this than I thought, and have made my library into a charm against mortality. As long as I have a few unread books beckoning to me from across the room, I tell myself I can always find a little more time.