I worked at a used book store years ago, and one of the things I learned quickly was that it's okay to destroy and throw out books. We used to have many, many copies of certain popular books, too many to resell. We bought them for very little, so we'd end up pitching a lot of them. For paperbacks, the easiest way to make a book unsellable is to strip it: rip the front cover off and throw the book itself in the trash. Since we were a used book store, we also trashed the covers; new book stores return the covers to the publishers for credit.
But many people bringing books in to sell to us would bring us books that were already stripped, or falling apart, or water-damaged, or otherwise virtually unreadable. Sometimes the person would say, "I found these old books that someone just threw away," with the same air of pride that you'd expect from someone who'd just rescued a nest of kittens from being eaten by trolls.
People value books way more than they value even comparably more expensive household items. Not very many people would see an obviously broken-past-repair toaster oven on the side of an alley and pick it up, thinking indignantly, "Who threw away this toaster oven?" Many people actually get angry at the thought that a book could be considered disposable--read it once and throw it out.
We're taught to respect books as small children, when library books are vulnerable to clumsy little hands, and in school when textbooks are too expensive for schools to easily replace. But I think there's more to it than just that. Most humans respect books because they're repositories of our collective knowledge. Even showing disrespect for specific books takes on a ceremonial air. Hate groups don't hold book-trashings, they hold book-burnings. The wrong knowledge has to be destroyed by fire.
But please, people, respect your local used bookseller. Don't fish books out of the dumpster and try to sell them. Because that's just gross.