Why is book not in accusative case here? Isn't kitap a definite object here?
It is not. It is the object of a postposition. When nouns have a postposition, you have to learn the case that goes with it (with hakkında, it stays in the nominative)
Because some prepositions change to the genitive case if combined with pronouns
Dersin is more like what would you say than diyorson which is more like what do you say.