Six Not-So-Easy Pieces is Richard Feynman's follow-up to Six Easy Pieces. Both are based on Feynman's lectures to undergraduate physics students at California Institute of Technology (Caltech). Six Not-So-Easy Pieces can be found both in audio and printed format.
The six not-so-easy pieces covered are vectors, symmetry in physical laws, the special theory of relativity, relativistic energy and momentum, space-time and curved space. Although the editors of the lectures (and later the books) decided to break the work down into six pieces, it should be noted that Feynman's style is such that each piece builds on the next. Many of them build on his work in quantum electrodynamics, which won him the Nobel Prize in Physics in 1965.