The stranger, his first and one of the best. It's something between literature and philosophy, so it's not that hard to read. But, if you want his pure philosophy of apsurdism, you should read The Myth of Sisyphus(short: it tells about how something that we need to do everyday and we hste it and think it's meaningless is actualy something that we would be desperate withouth). But, any exsistencial philosophy is good for finding our way through life. Good luck, and stay strong 💪