The purpose of your essay is determined by your goal as a writer, as well as your subject matter. There are numerous types of essays, but essays usually fall under one of the following purposes for writing: to inform, persuade, explain or entertain.
Here's what I found on the web I hope it helps you
First Question: http://www.sparknotes.com/lit/uncletom/themes.html
Second Question: http://www.sparknotes.com/lit/uncletom/themes.html
Well, a author has many types of styles, but it all depends on what author you are talking about.
