Plato's works are very readable. Since he was the father of philosophy, there was no technical vocabulary at the time. He wrote in the form of dialogues, so the prose is less dry than mere treatises. Aristotle is a little more difficult, but he is still very systematic and clear as long as you can pay attention. Mortimer Adler is one of the clearest and most common-sensical writers in philosophy. St. Thomas is not easy to read, but he is the essential Catholic philosopher. Popes, Church Councils, philosophers, and theologians have all sung his praises. The Bible (start with the New Testament if you haven’t already) and the Compendium to the Catechism of the Catholic Church (It’s the short version – or you can go for the Catechism itself if you are feeling ambitious). God Himself is truth, and philosophy is the path that leads to Truth.