A piece of E=mc2...

E=mc2: A Biography of the World's Most Famous Equation is a good read.
Maintains a fine balance between science and history without dwelling too much into details of both!
It's not just about Einstein or E=mc2 .Rather it explains in simple words what led(read, inventions and lives of other scientists) to the equation E= mc2 and then what the equation led to.

This piece best describes the mood of the book.

What guided Einstein was that, in his mid-twenties he found the unknown intriguing. He felt compelled to comprehend what might have been intended for our Universe by The Old One(as he referred to his notion of God).
"We are in a position," Einstein explained alter, "of a little child entering a huge library, whose walls are covered to the ceiling with books in so many different languages.
The child knows someone must have written those books does not know who or how.It does not understand the languages in which they are written. The child notes a definite plan in the arrangement of books, a mysterious order, which it does not comprehend but only dimly suspects."