Often we, as readers, find ourselves being handed the ideas and subsequent lessons that an author wants to express in his or her fiction. A book that sets itself up to teach the reader certain facts, morals, reactional analysis, is leading the reader to the conclusions that are already evident in the book. It is like an affirmation of what the writer has expressed, not too much has to be thought of by the reader to "get it".
In To Kill a Mockingbird, we are reading Harper Lee's interpretation of what Scout, a young girl, sees and reports on. She is almost omniscient in the fact that she is blatant and inexperienced in how she should react to the happenings around her. We, as the readers, need to fill in the blank of how she could, or should react. We are free to observe the action through her eyes and can react from our own core of knowledge as well. We can "think" about the plot and make our own judgements as to the plausibility of the action, we don't have to rely on Scout's ability to write it all down.
Many book are written in such a way that the audience is told exactly how the protagonist changes and exactly how all events are to be interpreted. The Scarlet Letter, by Nathaniel Hawthorne, is a great example of this. The readers are told how each character learns his/her lesson. In this book, however, that is not the case.
Take the narrator, to begin with. A book that thinks for you will most often have an outside narrator who explains the actions of each character and what he/she is thinking. However, in Mockingbird, we have a young girl as our narrator. Scout doesn't understand much of what is happening around her. For example, she doesn't understand at the courthouse that the men are there is raid the jail and lynch Tom Robinson. The readers must infer that this is their purpose from the discussion they have with Atticus. Scout also does not understand why the men go away. We, as readers, must understand that seeing Scout has caused Mr. Cunningham to think of his own son, and to be ashamed of what hs is planning to do.
Although Atticus provides some clear lessons for Scout, such as it is a sin to kill a mockingbird, it is the readers that are left to make the connections from this message to Tom Robinson to Boo Radley and beyond. Through the eyes of a child, we see the world in which she grows up, and are left to draw our own conclusions.