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 "Proo

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.