You Died Symbol Reference: Holy Symbol of the Sun
Both the You Died hardcover edition and paperback edition feature the Holy Symbol of the Sun.
In Dark Souls this symbol is most prominently featured on the Armor of the Sun and the Sunlight Shield used by Solaire of Astora. But their item descriptions mention, that these were “painted by Solaire himself”.
So where did Solaire get the idea for the Holy Symbol of the Sun?
A slightly different version of the symbol is used by the Warrior of Sunlight covenant as can be seen on the Sunlight Medal. This medal is (presumably) handed out by the covenant to players who summon one of their members and beat a boss with them. So it stands to reason that the covenant created the symbol. (“The symbol represents Lord Gwyn’s firstborn, who lost his deity status”).
The covenant itself is represented in-game by this symbol. It looks similar enough that it was probably the color template of the symbol minted onto the medal.
As Solaire is a member of the covenant he apparently wanted to show his allegiance.
In the absence of an official suit of armor, he chose to take matters into his own hands and painted the covenant symbol onto his shield and armor.
As awesome a warrior he might be, apparently his line-work is not on the same level as his fighting skills so in the drawing process he modified the symbol slightly resulting in the version we all know and love.