The title of this book probably scares off many readers, but it need not do so.  This book is a sequel to Feynman's "Six Easy Pieces".  Both books consist of lectures taken from Feynman's three volume lecture series, which was used for a two-year introductory physics course at Cal Tech.  "Six Not So Easy Pieces" is not as easy as "Six Easy Pieces".  The latter is accessible to most high school students, but the former is not. The not so easy pieces are:<br>· Vectors<br>· Symmetry in Physical Laws<br>· Relativistic Energy and Momentum<br>· Space-Time<br>· Curved Space<br><br>"Six Easy Pieces" was a bit unfocused because it covered a number of different topics.  In contrast, "Six Not So Easy Pieces" is focused on just one topic, Relativity Theory.  Feynman uses each topic to build up to Einstein's theory of gravitation, which is not the easiest subject to grasp.<br><br>Relativity theory is generally covered in one of two ways.  Some books give a general treatment, focusing on the many implications of the theory, with no mathematics.  At the other extreme are texts that require considerable knowledge of matrix algebra and differential equations.  This book takes a middle ground.  There is some math, but only some simple algebra and vector analysis.  (The vector analysis does go beyond what is taught in high school, venturing into 4 dimensional vectors, but Feynman shows this to be only a modest extension from the typical 3 dimensional vectors.)  Feynman dispenses with the usual introduction to relativity theory that employs moving trains and lightening strikes.  Instead he starts with a superb analysis of the Michelson-Morley experiment (alone this is worth the price of the book) and the Lorenz transformation that was developed to explain the null result that Michelson-Morley obtained.  He then uses this as the starting point for the analysis of special relativity.  This is followed with a discussion of relativistic energy and momentum, subjects that are generally left out of "popular" treatments of relativity theory.  The final chapter discusses curved space and Einstein's theory of gravity.  These topics will be hard for people without some basic physics background, but are not too advanced for the intended audience of college undergraduates, although Feynman admitted that only the best students did as well as he had hoped.<br><br>As with all of Feynman's books there is a wealth of knowledge packed a small package (in this case a bit less than 150 pages) and while these six lectures are not a substitute for a complete course in the special and general theories of relativity, it does hit the highlights and as usual provides very valuable insights for readers with all levels of knowledge.