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.