Before I forget! The Emperor is not the one Ridcully should divine. Or rather, he is not the most important person for Ridcully to divine.
It's the shamans. Ridcully should divine the shamans.
The shamans were the best psykers. They far exceeded the eldar. They created the Emperor, which was a psychic feat that far exceeded anything the eldar had ever done in the entirety of their history, and they did it with practically zero technology and resources.
Most importantly, they did it all as ordinary humans. That means we can learn from them. There won't be issues like with Areatha or the Emperor or the eldar, where their natures are too dissimilar from our own to gleam anything useful from them. The shamans were just like us, so everything they knew would be comprehensible to us.