One of the most quietly beautiful offshoots from the regular DC continuity, All-Star Superman is known not only for having one of the most iconic opening pages in the history of the comic medium, but for being a perfect treatise on why Superman is such an important and enduring character.
The story follows the last hours of Superman after he is poisoned by an overdose of solar radiation. Stronger than ever but with numbered days, he does his best to prepare himself and his loved ones for his departure.
The comic climaxes with one final confrontation with Lex Luthor, who has stolen a chemical recipe that lets him copy Superman's powers. After finally getting to feel superior to the Kryptonian, Luthor has an epiphany as he starts seeing the world from Superman's perspective, how all life is bonded at a molecular level and therefore everything is connected.
As his body begins to disintegrate, Superman flies off towards the sun, which Solaris had turned blue. As the humans look on below, the sun turns yellow again. Much later, Lois and Jimmy Olsen reconnect in a park. During their talk, Lois states that Superman isn't gone, he's just fixing the sun. Jimmy and readers are left broken by her words, unsure if she could not come to terms with his death or if she knew something they didn't.