Relativity is one of the two central perspectives of contemporary physics, and raises all kinds of interesting questions about space, time, causality and perspective. Einstein's engaging and accessible book remains an excellent short explanation.
Part of Einstein's scientific genius was his ability to bring about conceptual revolutions by simplifying existing theories in brilliant but unexpected ways. He had a gift for clear thinking, and that gift shows up in his writing for general audiences as well.
Relativity was first published in English in 1920, but Einstein later added five appendices, the last of which first appeared in 1952 (15th edition). Since this appendix is still under copyright, most current editions of the book do not include it. For Einstein's complete text you need the Routledge Classic paperback, available on Amazon for $17.95.
Also highly recommended is the Masterpiece Science edition, which is missing the last two appendices but includes a contemporary introduction by renowned physicist Roger Penrose, a commentary by University of Chicago physicist Robert Geroch, and an assessment of relativity's cultural legacy by historian David C. Cassidy—all for $6.99 on Amazon (paperback).
If you don't need all the appendices, there are several Kindle editions starting at $0.99, and it's free on iBooks.
Robert Geroch. General Relativity from A to B. Chicago: University of Chicago Press, 1978. 240 pp. (Dr. Mirus)
Delo E. Mook and Thomas Vargish, Inside Relativity. Princeton: Princeton University Press, 1987. 322 pp. (Dr. Sullivan)