Don't get me wrong: there are some books I have attempted that I ended up finding indescribably dull. But some of them, probably most of them, would have been redeemable to some other reader. I don't think there are many books that are totally, irrevocably, objectively boring. Even if a book is badly written, there is almost always some sort of amusement to be had from it - if only the kind of amusement derived from laughing hysterically over the sheer badness. Other books have been written for a very small niche, and people in the niche find them fascinating. I wouldn't enjoy a book on the different kinds of amoeba or the habits of the triple-eyed, red-spotted hairy antelope (actually, if there were such a thing I would be very interested), but others consider such works the cat's meow. As Anna quoted just the other day in a different context:
there are no uninteresting things, only uninterested people.
We ought to be careful, I think, before allowing ourselves to be bored by a book. We are far too ADD in the 21st Century; why else would authors be instructed to have a "catchy" first line and to be sure their story grabs the reader's interest in the first chapter? As readers, we are no longer willing to give the book our attention - it has to grab our attention. And if it doesn't do so quickly, we tend to put it aside and pick up something more in line with our tastes.
I don't like to let myself not finish a book, generally not because of any well thought out reason, but because it goes against my grain. Sometimes I do set one aside; just recently I tossed away a book I had been reading for research, highly disgusted with its lack of helpfulness and the author's obnoxious use of the word "hegemony." But most of the time I stick to the book with a kind of grim will, while a series of thoughts run through my mind. I start out by telling myself, "Maybe it will get better," and that takes me through about half the book. At that point I lose hope, but start telling myself, "I've gotten this far, and I'm just too stubborn to quit!" That gets me three quarters of the way through. Then, if the book still hasn't picked up, I've stopped being at all hopeful and started being desperate, but can't bear to give up so near the finish line. That would be like the blonde who swam three quarters of the way across the Channel, got tired and swam back. (My apologies to all blondes!)
All of that to say that as we read, we should be cautious of our opinions, considering them closely and not cementing them too soon. If a book is neither dirty nor mere drivel, we ought to give it time to develop before "pronouncing an adverse judgment," as Mary Bennet would say. If it is outside our usual range, good: we might find we like these new stomping grounds, and if not, we can at least have a glimpse of how they look. If we find the style or language trying, fine: our brains can always use a bit of exercise with wrangling out the meaning of Shakespeare. If the book is huge, it's good practice for keeping our minds engaged - and besides, the feeling of success is greater in the end.
None of this is to say that we should never put a book aside as long as it isn't obscene. But I do think we ought to consider why we're not finishing it, and be able to give ourselves a good reason. We should not let ourselves turn away for the mere trifling reason that a book seems "boring." Perhaps the real issue will turn out to be not that the book is uninteresting, but that we are simply uninterested - and the book might even be one that we would do well to make ourselves be interested in.