Empire by Clifford D. Simak

A miracle came to pass in Ranthoor when a man for whom all hope had been abandoned suddenly appeared within the city's streets. But he appeared to be something not quite earthly, for he did not have the solidity of a man. He was pale, like a wraith from out of space, and one could see straight through him, yet he still had all the old mannerisms and tricks.

In frightened, awe-stricken whispers the word was spread. the spirit of John Moore Mallory had come back to the city once again. He bulked four times the height of a normal man and there was that singular ghostliness about him. From where he had come, or how, or why, no one seemed to know.