My Superman recommendations continued:
Superman in World's Finest Comics Archives volumes 1 and 2.
Superman in the Fifties.
The Superman Encyclopedic by Michael Fleisher is a great source of facts, tells a wealth of information about the Golden Age Superman and the Silver Age.
Jimmy Olsen Adventures volumes.
Jack Kirby's Fourth World #20 (1998) "The Gods Themselves."
Legends of the DC Universe #14 (1999) "The American Evolution."
Superman #247 (1971) "Must There Be a Superman?"