Probably my favorite classic thus far. Much like The Myth of Sisyphus, the Meditations on First Philosophy have somewhat of a pop-philosophy following thanks to the 'I think, therefore I am' principle. However, I think these essays go so much further into the very interesting concept of what can be known and what can not. Though he makes his argument as a case for the existence of God, it still remains a series of exceptionally interesting thought experiments (and some say he was actually areligious). Highly recommend as an introductory philosophy book.
If you liked this, you might also like Plato: Five Dialogues
A solid classic that is definitely worth the read. Surprisingly, this book wasn't as difficult of a read as I expected it to be, so props go out to the translator (Grube). Each dialogue covers a different moral or ethical topic. Of the five, I found Apology to be exceptionally monotonous but Phaedo to be very memorable (mostly thanks to the tragic nature of Socrates death). If you're a fan of philosophy, this is a must read.
If you liked this, you might also like The Stranger
A great read if you are especially interested in the philosophy of existence. In 'The Myth of Sisyphus', Camus attempts to rationalize human existence on the basis of the absurdity of existence itself. While it was a very interesting read, the classical nature of these essays paired with Camus French background made for a relatively difficult read. Besides that, Camus and I disagree on issues of morality & ethics. Regardless, a good read for philosophy lovers or those that have ever struggled with thoughts of suicide.
If you liked this, you might also like The Stranger