“Relativity The Special and General Theory” by Albert Einstein is great. Translated by Robert Lawson. It the 1920 translation. It’s a free book online.

XII. The Behaviour of Measuring-Rods and Clocks in Motion

http://www.bartleby.com/173/12.htmlSqrt(1-v2/c2) -

and for still greater velocities the square-root becomes imaginary. From this we conclude that in the theory of relativity the velocity c plays the part of a limiting velocity, which can neither be reached nor exceeded by any real body.

Of course this feature of the velocity c as a limiting velocity also clearly follows from the equations of the Lorentz transformation, for these become meaningless if we choose values of v greater than c.

Because with v greater than c; 1-v2/c2 is a negative number and there can be no square root of a negative number.