If we are talking about valuable books, then content can be recovered at least partially from a wet book, depending on how good the printing ink and paper were and how long that book stayed under water.

Fire is more dangerous than floods and historically we've lost more books by fire than from anything else, although I'm pretty sure there are effective methods to recover content partially from books that are only partially burned.

The durability of physical books is amazing and the only thing going for digital content is that the cost of copies is zero, which means you can easily make backups in multiple places of this world instantly and retrieve those backups with whatever device you happen to have in front of you. Unfortunately that's exactly what DRM is trying to prevent.

Also bugs.

Also rodents.

My 15 month old daughter has started being a little destructive to books as well.

So aside from fire, water, rodents, bugs and children, what could possibly happen to a physical book?

Other than loss and theft of course.

Dust, and dust removal, and dust prevention, present risks to old books.

(http://www.nationaltrust.org.uk/what-we-do/what-we-protect/c...)

(http://www.nationaltrust.org.uk/servlet/file/store5/item3780...)

Some oppressive regimes will confiscate some books. I don't know what happens to that book afterwards.