Wow. This is a great question!! Lemme see.... hmmm.... the classics, of course. And as to the "why", I think reading just makes you more of a well-rounded person, opens your mind, helps your grammar, just really opens up your world and imagination. I wish more kids read books, and spent less time on computers and TVs and video games. Books educate you, no matter the book.
But, as to which specific books.... I am not sure, I do think there are some classics that just really must be read, for the simple fact that they are such great novels...such as "Tom Sawyer", "Black Beauty", "Robinson Crusoe", "Last of the Mohicans"...... so many more...