QED

Knowing absolutely nothing about quantum electrodynamics (the theory of photon interaction with matter), one can still make sense of the the book - and that is Feynman's true genius, in a way.
In a nutshell, QED is (at present) the nearest step to a Grand Unified Theory, the utopian theory which will explain the universe in the form of formulae that are so short that they can be inscribed on your shirt. It explains almost everything that we notice in our day-to-day lives - ranging from why you see your image in a mirror, to more esoteric effects such as predicting which particles arise during extremely high-energy collisions (such as in the Large Hadron Collider).
It is a marvel to witness a genius at work. Without any Math whatsoever, Feynman explains the theory while not leaving out any detail - where it works, where it doesn't work, and how our understanding of Physics is still pitiful, because every answer that we find brings a whole lot of questions to the fore.
The fourth chapter is on our understanding of Physics so far, and where QED fits in - it is a tonal mess, especially when compared to the succinct previous chapters, and it is the only time I felt disgruntled when reading the book.
TL;DR - an amazing read to reinvigorate interest in Physics/science, or to ignite the spark of interest that was never there in the first place. Either way, it is a must-read for the inquisitive mind.