Slouching and cramped books will become deformed and damaged. Their bindings will deteriorate and they will become very difficult to use without causing more damage. If books are haphazardly sprawled on shelves, both in the stacks and public access areas, the next person to use them is also likely to be careless about handling them, but if they're upright and neat, it's most likely they will be kept that way. Photocopying can also be very damaging Pressing down on a book as hard as possible will almost certainly break the spine. If you look like you are treating something with respect others might do the same, so the less careful you are, the more other staff and readers are likely to follow.
Books love to treated with care and respect.
They can provide you with a lifetime of enjoyment. So remember to set a good example and avoid being a book abuser.