This Wikipedia entry has more details. But do read this excellent, very human, Superman story first.

Book 2 continues with Superman plunged into different adventures (meeting his Kryptonian replacements; finding his way out of the Underverse…)

He of course completes his 12 Challenges. The beauty is in the telling. The takeaway from reading this was a feeling that Superman really existed, and that the really died and that’s why we don’t see him anymore.

Fantastic storytelling and beautifully pencilled and illustrated.