Prelude to Foundation

Hari Seldon proves that psycho-history can exist. He just doesn't know how it works. He is sent on a quest to find how to make it work. 
He thinks he needs to look at the origin of humanity, and his adventures brings him all around the capital/planet of the Empire: Trantor. 
What he comes to discover is that Trantor is a microcosme of the human empire, and is enough of a sample to establish the rules. 

 I had read it when I was 16 and kinda remembered the ending. I thought I might get at what kind of rules Asimov had in mind, but he stayed both close and far from what the rules would be. Anyway, made me want to read more, like the first time