You sound like a real bookworm, like I am, no movie could compare to book form, I can't think of any examples of movies better than their original novels. Movies popularize the novels though. Few people take the time to read an entire novel. Movies can breathe life into a novel; the challenge of maintaining this depth of experience throughout an entire film is the directors. Movies are still a budding art.
Part III dosen't take you in the direction you want just for fun. Its the fall of the 'Roman Empire', conventinal morality as we know it. Happiness is the elusive object, the Corleones have the money when will they get the respect? Why are they driven to seek more power when the power they have has cost them so much already? Why did Don Vito order the hit on Woltz with such magnamity? The one guy popular enough for it to back fire. Did Don Vito know word would get out?
Is the Godfather about being the toughest, the richest, or the smartest? Or, beter yet the most moral, from which these other things may be included. What price did Don Vito know he would pay in is struggle for justice. It must of felt good to stick that knife into Don Ciccio, finally the man he saw murder his own mother. The story is a soap opera functioning on a literal reality plane.