@Catmae : That's why during Harvey, when several of the libraries flooded, there was a huge rush to get the books on the second floor, that hadn't flooded, out of the building. Their proximity to that water, no climate control in the buildings, would have ruined them. And let me tell ya, mold/mildew is contagious! If you put a book that has mold/mildew or even just water damage, in it on a shelf with other books, it will eventually spread to the other books. And just because you dry a book out, doesn't mean it won't mildew. Once it has been exposed to water, well, all bets are off, unless you use that expensive process done to them. I believe it's almost like baking them, but safely, with no flame. I have also heard of freezing them.
If you're dealing with just your personal collection, you should be ok with a book you dried out. When you're dealing with thousands of books in a collection, water is your enemy.