In writing a book review, first make an evaluation of the work as a whole; that is, how it has affeced you. For instance, you may wish to classify the novel and then explain how it fits into this classification. In evaluating The Scarlet Letter, you may wish to focus upon the novel as a psychological analysis of the effects of sin. Then, you can divide your analysis into parts such as the external reminders (settings) of sin in the Puritan community such as the prison, the scaffold, and Hester's isolated cottage, and the effects of sin upon the individuals, Reverend Arthur Dimmesdale, Hester Prynne and Pearl, and Roger Chillingworth.
So, a thesis on a review of The Scarlet Letter could be something like this:
A psychological study of the effects of sin in a Puritan setting, Hawthorne's The Scarlet Letter focuses upon three characters in particular and their own brand of sin.
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One mistake students often make is trying to write a thesis statement to early in their writing process. Before you develop your thesis for a paper, you need to read the primary work carefully, and then decide what major theme of point seems to be most important or central to the work. Once you have decided what theme to focus on (e.g. adultery, shunning, hypocrisy, religious intolerance), you should write a detail outline of how you will approach that theme. Once you have your detailed outline, develop a thesis by making a specific claim about your theme and a statement about how you will approach that theme.