See books by Martin Gardner. This is available as an eBook:
Relativity Simply Explained
http://www.amazon.com/Relativity-Sim...martin-gardner
Full disclosure is that I haven't read the above, but I did like this seemingly similar title by the same author, not available as a proofread eBook, but only as an uncorrected scan:
Relativity for the Million
https://openlibrary.org/works/OL4600...or_the_million
P.S. When I go into the above link, I see at the top of the page: "Hello! The Commonwealth of Pennsylvania is participating in our eBook lending program." I'm therefore not sure it is generally available.