READER recently wrote in to say she was ''amused -- vindicated in fact'' -- to discover in this column some disparaging comments about a certain historical novel she'd read. ''I knew,'' she continued, ''like a bad marriage, that I had made a mistake halfway through the first chapter. I was bored, irritated and actively offended,'' but ''brought up in New England on various strictures of the clean-your-plate school, I did skim it from start to finish with no change of heart.''

Yankee resolve is justly celebrated, but this is over the top. Why subject yourself to an irksome book when so many sublime ones are available? Nevertheless, every reader recognizes the threshold my correspondent has yet to cross: the moment when you decide that you don't have to finish every book you start.

For some, it's like a loss of virginity; you never forget the book that defeated your naive faith in the contract between an author and his or her reader, the promise that your time and effort, even your irritation, will be fairly repaid. (In my case, it was ''A Portrait of the Artist as a Young Man''; I had had about enough of Master Dedalus, thank you very much.) For anyone who reads books professionally, that faith dissipated long, long ago, and even the perversely principled stick-to-itiveness that makes a person gut out a book that reminds her of a badly chosen spouse has become a distant memory. Even critics who start out as hungry readers, devouring fat volumes in single, 10-hour sittings, learn to nibble, sampling a chapter at a time from each of the dozens of new books that arrive in the mail every week. It's a warped, unnatural way to read, dictated by uncommon circumstances. ''I now finish no book I start,'' says David Gates, a novelist and critic, ''unless I'm reviewing it. Or if it's wonderful fiction, but I haven't seen a wonderful novel for a long time.''

But surely authors, who aren't responsible for filtering through piles of new releases and who know what it's like to pour years of work into a book that people will pass over with a glance at the cover and the jacket copy, are more generous? Not really. ''I'm very unforgiving,'' says Michael Chabon, the author of ''The Amazing Adventures of Kavalier & Clay.'' If the book doesn't grab him in a page or two, he's out of there. ''I guess I'm less responsible to books than I should be, but my time for reading is so limited and the competition is so fierce. It's a Darwinian process.'' Being perfectly willing to bail out when the going gets dull is, for many writers, less a matter of lost idealism than an apprehension of mortality. ''As time goes on, I'm more apt to abandon them,'' Diane Johnson, author of ''Le Divorce,'' writes via e-mail. ''I quite often lose books, leave them on buses or whatever,'' which she interprets as her unconscious relieving her of a duty when her conscious mind is playing the martinet.

Even younger writers feel the press of time. Myla Goldberg (''Bee Season'') tells herself that reading a mediocre book ''would mean that I would eventually be on my deathbed having been deprived of the opportunity to read some other book, perhaps one that would have been really fun, or exciting, or even life-changing.'' Chabon gives a book two pages, Goldberg allows it 15 to 50, and a book editor I know says that ''publishing turns you into a person who decides within five pages whether you'll like something or not and who puts it down (whether it's work or personal reading) without one ounce of guilt if the answer is no.'' She added, ''I know someone who swears by nothing more than the first sentence.'' What puts these readers off? The most complained-of quality is ''lyricism,'' the piling on of metaphors, similes and extravagant imagery. Also hated are long passages of description (particularly of weather and geology) and hokey framing devices like ''I remember well the summer I turned 14. . . .'' For the writer, the pitfalls are many, and one imperative rules: ''Your beginning better be just killer,'' Chabon says.

Some might see this as evidence of a culturewide case of literary attention-deficit disorder, but it's hard to justify time wasted in the reading of unloved books. The burden is on the author to prove that what you're holding is something exceptional, and if not in the first few pages, then where? It's also unwise to idealize the passionately committed reading habits of youth; becoming a writer yourself can make you realize how low you once set the bar. ''I had an insatiable appetite for complete narratives,'' says Jonathan Lethem (''The Fortress of Solitude''), remembering the years when he finished every book he started. ''I needed to know what happened. I'd fillet a novel of its story. Now I read more slowly, less to get to the end than for the pleasure of the sentences and paragraphs. Before, it was pure consumer frenzy.''

Others described their need to read to the end of even the worst book in similarly pathological terms: ''an obsession,'' ''a sick sense of loyalty,'' ''masochistic.'' Ayelet Waldman, a novelist (''Daughter's Keeper'') who is married to Michael Chabon, claims to have ruined a family vacation in Hawaii because she refused, with a tenacity her husband found maddening, to jettison a book she loathed. ''The rage that it engendered kept me going,'' she says. ''I have to feel personally betrayed by a book to quit, but sometimes, exactly like some relationships I've had, the betrayal becomes so catastrophic that I keep going back to it.'' Most will persevere with a trying book only if it comes highly recommended. ''It's like dating,'' says Tom Bissell, the author of ''Chasing the Sea.'' ''You need to know if this is serious or just a fling.''

The editor Robert Gottlieb, a prodigious reader, maintains that he never deserts a book, although closer questioning reveals that it takes him quite a bit of ''reading in'' a volume to decide that he's started it. Gottlieb, who says he'll sometimes read an old, forgotten book just because ''I feel sorry for it,'' also believes that he'll get back to the many partly read books in his life eventually, which makes them half-finished, rather than un. The distinction is fine, but useful; by the time fate obliterates the difference, you won't care.