This is not a traditional layman's book as it does assume some background in calculus and physics -- but it is not overloaded with endless mathematical equations, just the critical ones. If it's difficult to appreciate the meaning of an equation like F = d(mv)/dt -- then it's probably best to skip the book -- but if it resonates with you, the book is very approachable. Though his style is informal, Feynman's words are carefully chosen.  It is not especially long -- but it should be read very carefully.  The book contains excerpts from his famous lecture series -- this particular collection focusing primarily on special relativity with some vector math, symmetry, and a touch of general relativity tossed in.  If you have a layman's working knowledge of special relativity, this is a nice way to cement it into place with a very intuitive development from a moderately basic level.