What Is Relativity?: An Intuitive Introduction to Einstein’s Ideas, and Why They Matter by Jeffrey O. Bennett
Targeted (presumably) for high school and early college students, this is about the clearest explanation of the implications of relativity I’ve come across. Many of the thought experiments he uses are familiar (clocks, rubber sheets, etc.), but his talent for clear language make them seem almost intuitive. For an easy introduction to a difficult subject, you can’t do much better.