Philosopher A.N. Whitehead famously said that the European philosphical traditino is a "series of footnotes to Plato." Hyperbole, perhaps, but there is no doubt that Platonic thought is fundamental to philosophy. Boston College professor Peter Kreeft discusses the content and repurcussions of Platonism in a series of lectures, The Platonic Tradition.
Kreeft is a great popularizer of complex ideas. The Platonic Tradition is a bit heavier than some of his other works, such as his dialogues, but he definitely makes philosophy accessible to the non-philolospher.
As a Christian, Kreeft emphasizes the impact of Platontism on Christian philosophical and theological traditions more so than, I would suppose, a secular philosopher might. But the strongest part of these lectures is Kreeft's descriptions of Platonism in Augustine, Aquinas, and other Christian thinkers. On a different note, he attrubutes the more recent philosophical movements like nihilism and existentialism to the abandonment of Platonism.
Kreeft is dealing with bid ideas in The Platonic Tradition. But he makes them approachable by providing context and multiple layers of explanation and illustration. Highly recommended.
2016 Reading Challenge: A book about philosophy