Italo Calvino gave the best explanation in his 1979 novel If on a winter’s night a traveler (in Italian: Se una notte d’inverno un viaggiatore). A pre-kindle, pre-Amazon, pre-self publishing analysis that’s still incredibly valid today, and confirms how Calvino was one of the most brilliant authors of the last century.
This is translated from the Italian by William Weaver.
“In the shop window you have promptly identified the cover with the title you were looking for. Following this visual trail, you have forced your way through the shop past the thick barricade of Books You Haven’t Read, which were frowning at you from the tables and shelves, trying to cow you. But you know you must never allow yourself to be awed, that among them there extend for acres and acres the Books You Needn’t Read, the Books Made For Purposes Other Than Reading, Books Read Even Before You Open Them Since They Belong To The Category Of Books Read Before Being Written. And thus you pass the outer girdle of ramparts, but then you are attacked by the infantry of the Books That If You Had More Than One Life You Would Certainly Also Read But Unfortunately Your Days Are Numbered. With a rapid maneuver you bypass them and move into the phalanxes of the Books You Mean To Read But There Are Others You Must Read First, the Books Too Expensive Now And You’ll Wait Till They’re Remaindered, the Books ditto When They Come Out In Paperback, Books You Can Borrow From Somebody, Books That Everybody’s Read So It’s As If You Had Read Them, Too. Eluding these assaults, you come up beneath the towers of the fortress, where other troops are holding out:
the Books You’ve Been Planning To Read For Ages,
the Books You’ve Been Hunting For Years Without Success,
the Books Dealing With Something You’re Working On At The Moment,
the Books You Want To Own So They’ll Be Handy Just In Case,
the Books You Could Put Aside Maybe To Read This Summer,
the Books You Need To Go With Other Books On Your Shelves,
the Books That Fill You With Sudden, Inexplicable Curiosity, Not Easily Justified,
Now you have been able to reduce the countless embattled troops to an array that is, to be sure, very large but still calculable in a finite number; but this relative relief is then undermined by the ambush of the Books Read Long Ago Which It’s Now Time To Reread and the Books You’ve Always Pretended To Have Read And Now It’s Time To Sit Down And Really Read Them.”