The opening of the novelization sheds a lot of light on the original concept of the empire, the powers the moffs had and what the emperor originally was.
This is from the prologue:
Another Galaxy, Another Time.
The old republic was the republic of legend, greater than distance or time.
No need to note where it was or whence it came, only to know that ... it was the Republic
Once, under the wise rule of the senate and the protection of the jedi knights, the Republic throve and grew.
But as often happens when wealth and power pass beyond the admirable and attain the awesome, then appear those evil ones who have greed to match.
So it was with the Republic at it's height. Like the greatest of trees, able to withstand any external attack,
The Republic rotted from within though the danger was not visible from the outside.
Aided and abetted by restless, power-hungry individuals within the government, and the massive organs
of commerce, the ambitious Senator Palpatine caused himself to be elected President of the Republic.
He Promised to reunite the disaffected among the people and to restore the remembered glory of the Republic.
Once secure in office he declared himself emperor, shutting himself away from the populace. Soon he was controlled by the very assistants and boot-lickers he had appointed to high office, and the cries from the people for justice did not reach his ears.
Having exterminated through treachery and deception the jedi knights, the guardians of justice in the galaxy, the Imperial governors and bureaucrats prepared to institute a reign of terror among the disheartened worlds of the galaxy.
Many used the Imperial forces and the name of the increasingly isolated Emperor to further their own personal ambitions.
This is all but the entirety of the prlogue except the final 2 paragraphs about the rebellion.
Copywrite Lucasfilm 1976, and del rey 1976.
The Emperor here does not recall the emperor in the empire strikes back from 1980 or 1983 return of the jedi. in fact this reflects the original wizard of oz type nixonian character Lucas originally envisoned and no sidious the sith lord who is also chancellor/emperor palpatine.
In fact he almost sounds a bit like the emperor from the novel dune.
So originally the regional governors also known as the moffs would have been more powerful than the emperor himself which is why tarkin is more powerful than vader.