Who said "Atticus Finch’s a deep reader, a mighty deep reader."
You can find this quote in Chapter 16. However, you will not be able to find any specific name for the person who speaks it. We know it is a man, but we do not know his name.
The man who says this about Atticus is described as a member of the "Idler's Club." These are a bunch of old men who hang around the court house and watch trials. Usually, they are the only ones there.
A group of them are talking about Atticus. Most of them are saying relatively mean things about him. They are upset because he is going to defend Tom Robinson in a serious way. But the man who calls him a "deep reader" seems to be serious -- the description seems genuine and not sarcastic.