1) **** "Einsteins' Mirror" by Tony Hey and Patrick Walters 1997. The best all-round resource for special and general relativity. There are lots of pictures, diagrams, history, experimental evidence and depth to the discussion.
2) **** "Very Special Relativity" by Sandar Bais, 2007. The clearest treatment of Minkowski spacetime diagrams. It's all geometry and it's beautifully presented.
3) **** "Spacetime Physics: Introduction to Special Relativity" by Edwin F. Taylor and Archibald Wheeler, 1992. This is the most conceptually rigorous and thorough treatment of relativity for high school students. It has a little calculus but it isn't essential to following the discussion. Not an easy read, but well worth digesting slowly.
4) *** "Six Ideas That Shaped Physics: Unit R", by Thomas A. Moore 2003. A very clear textbook of special relativity which incorporates active learning strategies - a great source for spacetime problems.
5 ) *** "Was Einstein Right?" by Clifford M. Will 1993. A very clear explanation of general relativity and the tests that it has undergone.
6) *** "Black Holes and Time Warps: Einstein's Outrageous Legacy" by Kip Thorne 1994. An extremely well written explanation of General Relativity and its implications. It includes a lot about the history and personalities invovled in the development of our understanding of Black Holes. Not the simplest, but the most thorough popular book on Black Holes.