Superman has one secret identity. In most (?) continuities, Clark Kent is a pretence. In John Byrne's continuity, Kent was real; Superman was a role.
The Martian Manhunter can have many secret identities because he is a metamorph. Elliot S. Maggin's Lex Luthor has many false identities because he is a criminal genius who needs both to hide from Superman's super senses and to remain active in the world. This makes sense. Everything about Maggin's understanding of these characters makes sense.
Maggin's two Superman novels reflect in detail the comics continuity of the 1970s with, e.g., Kent and Lang on TV. In fact, the novels exactly fit into this continuity. Maggin even wrote sequels in the comics.
Maggin's best idea was that, in the future, Superman and the reformed Lex will again be friends. See here. Also good is Alan Moore's alternative account of these characters' futures. See here and here.
Byrne, Maggin and Moore: three great Superman writers -
Moore: the "last story."
Byrne: a new beginning.
Maggin: insightful novels.