Well, the evidence is primarily from classical Greece and and the cults in the various regions. But he does delve into earlier times (Indo-European, Minoan-Mycenean) as well as later times (Hellenistic, Roman, and even Medieval and modern interpretations).
I consider myself a well read pagan, but there were some details here that were new to me.
When there is no central body to define and enforce dogma, local cults can experience and envision a deity in slightly different ways.
But I think we are talking about different spins on on the same god, not different gods per se.
The root of any interpretation of Zeus is the Indo-European Shining Sky Father. Jupiter is based on the same god, which is why the two deities were so similar even before the Romans adopted Greek mythology wholesale.