The endpapers (which include credits and publishing information) are close ups of the raindrops from the first and last panel of the story. Story tells a comprehensive origin of the Joker based on the story "The Man Behind the Red Hood" from Detective Comics (DC, 1937 series) #168. This story is still considered controversial over two decades after its publication, mostly due to the crippling of Barbara Gordon (which was not revealed to be permanent until subsequent issues of Suicide Squad where she first appeared as the wheelchair bound Oracle), but there is a clear indication that creators Moore and Bolland originally intended this to be a non-continuity "final" Batman vs. Joker story when Bolland strongly insinuates in the afterward printed in Batman: The Killing Joke Special Edition (DC, 2008 series) #[nn] that Batman kills the Joker "off camera" in the last three panels of the story.
The "joke" is a gag that can be traced back in the comics of DC at least as far as Mutt & Jeff (DC, 1939 series) #6 (Fall, 1942) in which Mutt suggests that Jeff climb up a spotlight beam. Jeff refuses, worried that the spotlight will be turned off and he'll fall. Possibly not the first or only time the joke's been told.
Back cover shows a playing card featuring Joker on the top and Batman on the bottom.