The Wikipedia article is also interesting. <a href="http://en.wikipedia.org/wiki/Sylvester%E2%80%93Gallai_theorem" rel="nofollow">http://en.wikipedia.org/wiki/Sylvester%E2%80%93Gallai_theore...</a>
Béla Bollobás "The Art of Mathematics" has a very nice treatment of this problem (the history, multiple proofs) as problem number 33. And Aigner, Zeigler "Proofs from THE BOOK" also has a good treatment.