Socrates was the first of the great Greek philosophers, who then taught Plato, who in turn mentored Aristotle. Aristotle (384 BC – 322 BC) is recognized as one of the most important contributors to Western philosophy and thought; many of his ideas were accepted as dogma for 2000 years. He wrote on numerous subjects including: politics, logic, music, theater, physics, metaphysics, poetry, ethics, biology, zoology and was the teacher of Alexander the Great.
I am currently reading two of Aristotle's works including Nichomachean Ethics, which is his magnum opus on ethics. The name of the book comes from Nichomachus, Aristotle's son, and this text "is widely considered one of the most important historical philosophical works" (Wikipedia).
The second book I am reading is Politics, a book Aristotle wrote about the philosophy of human social structure. It was Aristotle, after all, who coined the phase, "man is a political animal".