This book is about the special theory of relativity. A secondary schooling is adequate to understand it, but reading it calls for some mental concentration as well as an ability for mental abstraction. The unsophisticated reader may, therefore, find it difficult and boring. Nevertheless, since the discourse is abundantly interspersed with general statements and sundry examples and analogies, and insofar as most statements of fact are declared but not proved, the book can probably be classified as popular science reading.