Evolution of the double-decker bus.
 

This is primarily about the Leyland Titan, but the history of the evolution and the various versions (including the competition) - as well as the people involved in the design - makes the story an interesting and informative read.

I was going to copy an extract, but there is so much worthwhile reading that I suggest you decide how much you digest:-

http://en.wikipedia.org/wiki/Leyland...cker%29#Origin