The Origin of Superman
It's hard to believe that Siegel and Shuster never actually wrote an origin story for Superman. The character was actually around for more than a decade before the first version of his origin story saw print. Much of what we know of Superman's origin was actually written by Batman co-creator Bill Finger.
It's been reprinted a lot of times, so you're no doubt familiar with the basics, but some of the finer points are interesting, including Finger's explanation for why Krypton exploded, what an old car looked like in 1948, and other differences like Kal-El being taken to a home for foundlings before he was adopted by 'John' and 'Mary' Kent and both of his new parents dying before he assumes the identity of Superman (as part of a deathbed promise to his adopted father).