I used to see this on people's shelves when I was a kid and always wondered what was inside the "three red books." Well the short answer is: everything. It's a great way to learn physics Feynman's way, which means very little problem solving but a lot of deep comprehension and a thematic approach to physics. Meaning: he shows you certain paradigmatic problems which illuminate the physical world and which you can use over and over again. Also he shows you advance peeks at more advanced science, which you will certainly not get in most undergrad physics textbooks. Caveat: Feynman is not so deep philosophically. His take on relativity is strictly for calculational purposes and even in quantum (his specialty) you will find very few deep philosophical insights into the theory besides just: "shut up and calculate!" He seemed to be allergic to mixing philosophy and physics but sometimes it is unavoidable.