What is the best book or video to grasp the Relativity Theory?

Go to the source. Einstein himself wrote the best book available for lay people.

I first read this book when I was around 14. I’ve re-read it twice since then. I didn’t get some of the math at first, but I got the concepts, and it was definitely a mind-bender. I never looked at a passing train quite the same way again.

I don’t like Einstein’s book, actually. I have two favorite books to learn Relativity. One is The Elegant Universe by Brian Greene. The first few chapters are on Relativity, and have the best explanation I’ve ever seen. The second book is Relativity Visualized by Lewis Carroll Epstein, (although I’m seeing now that that’s out of print.)

@Rarebear +1 for The Elegant Universe. Though it’s written as a popular science book, I found it to be a good read.

I would also recommend Theoretical Physics by Joos if mathematical formulas don’t scare you.

“Gravitation cannot be held responsible for people falling in love. How on earth can you explain in terms of chemistry and physics so important a biological phenomenon as first love? Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That’s relativity.”

Albert Einstein
US (German-born) physicist (1879 – 1955)

I’m with @IchtheosaurusRex on Einstein’s book. I read it when I was 16, and it was a real help because it just talks about the theory, rather than complicating it with all the other things they wanted to attach to it at school. It is written in a relaxed, logical manner that made it easy for me to understand.

Alternatively, the website hyperphysics has great explanations of each concept.

the best video that i know of is The Mechanical Universe and Beyond by Dr. David Goodstein of Caltech. It’s shown on PBS sometimes if you can find it.It is very expensive to buy unfortunately

I’ve read several. The best is The Elegant Universe by Brian Greene.