Spinoza (cool stoic ideas and ideas in metaphysics)
Descartes Meditations ("how do i know i exist")
Zen Buddhism (on your state of mind and how to view the world)
Epictetus (stoicism)
Gettier (challenges Plato's JTB theory of knoweldge)
John Stuart Mill (Utilitarianism)
Find some stuff about Hinduism too, especially Brahman
those are all pretty popular and really stuck out for me