Hmm... If you want philosophy, I'd say Plato's Republic, though good and a classic, is a bit difficult. Not in the message or the style, but some of Socrates's generalizations make me think he had a bit too much wine at that party.
If you don't mind Wittgenstein's terse format, or you are a math person, Tractatus Logico-Philosophicus is an excellent book.
Ayn Rand is very good as well. The Virtue of Selfishness is probably one of the more accessible books/collections of essays. Very interesting, especially if you are interested in the objectivist viewpoint.