which is without having it spoiled for you first. Of course, the books for which that is most true are generally not the books with the biggest literary value, but then, that's perhaps all the more reason to respect the "intended" way to enjoy the book. If the book you're reading does not have particular literary quality, and the author used spectacular plot twists as one of the main attractions of reading the story, it may well be the case that if you already know of the plot twists beforehand, you feel you might as well skip the book entirely. The Da Vinci Code comes to mind.

For better books with more literary value, obviously there will be more room for different readings of the book, and readers appreciating different aspects, and then spoilers may not matter as much - but many readers will still prefer not to get them. At the end of the day, every writer's intended audience has to be people who know nothing about the book except what they read in it, per definition, as there always has to be a first reader. So many people feel that they will get the most authentic experience, the closest to what the author intended, if they read without spoilers. There are naturally post-modernists among us who don't care what the author intended and don't think it matters, but I dare say that's a minority...