Which is why I found one of the Q&As especially interesting. Someone asked what impact mildew has on the value of a book. The response? "Throw the book out." Mildewed books are apparently not only unappealing, but an actual health hazard, particularly to people with respiratory sensitivities.
And the mildew can spread to other books, as described in this writeup on Biblio.com. As you read it, take careful note of the suggestions for how to store books so that they don't develop mildew problems. And if you're holding a fundraising book sale, consider this your official permission to simply toss any books with a bad smell into the recycle bin.