Avoid water: You already know that you don't want your books to end up in the water, but there is more to "avoiding water" than just making sure your books don't fall into the pool or the ocean; you also need to realize that humidity can wreak havoc on the shape of your pages, so you should be cautious even reading beside the pool or by the ocean!
Avoid heat: As is the case with "avoiding water," it is difficult to avoid heat altogether, as 'reading by the pool in the summer' is one of the quintessential 'best ways to read," but be cautious of heat - especially from leaving your books in a hot car - as this can severely damage the binding of your books.
Avoid dirt: Some people feel that dirt adds character to an old book, but too much dirt can make a book a throwaway (and can even make it unreadable), so be aware of where you are when you are reading, and always be aware of the sort of state your hands are in!
If you have not yet switched to ebooks (or have not yet switched altogether), it will be important that you adhere to these rules, and that you make sure anyone you loan your books to is adhering to these rules; after all, with books so rapidly switching to electronic format, your books just might be collector's items pretty soon!